Add starting bank charging to solar system

barclaybrown · October 2014

I have three solar panels wired in series to a Morningstar TS-MPPT-60. The panels add up to a nominal 90 VDC and 750 watts, charging a large (1500 AH) 12v battery bank. I would like to use this same solar array to also charge the starting batteries on the coach which are a 24 volt bank. I don't need to use a lot of current to charge the starting bank. Would I use something like a TS-45 which can handle the 90v solar voltage? It doesn't look like any of the smaller controllers can handle the high voltage. There is no need for MPPT to charge starting batteries, right, since they are seldom discharged much?

Also, if I did get a TS-45 for this use, can the M-2 meter I have monitor this as well as the TS-MPPT-60 I have now, by switching it back and forth? Or is there a better way to do it?

Or, is there a device I can add on from another manufacturer to charge this 24 volt bank from the same solar array? Ideally I want some kind of monitor or meter so I can see at least the voltage of the starting bank and perhaps the charge current

Thanks!

Cariboocoot · October 2014

Re: Add starting bank charging to solar system

Well let's see ...

First of all 750 Watts isn't going to charge 1500 Amp hours @ 12 Volts, so you don't actually have any power to spare. Really you should have over 2300 Watts for such a bank, and two charge controllers to handle the massive current. As it is 750 Watts will provide about 48 Amps peak, or roughly a 3% maintenance charge.

If this set up is 'working' for you then in all likelihood you either are running an additional charge source or are way over capacity in batteries.

As for running an additional charge controller from the same PV array to the 24 Volt batteries, that's not a good idea even if there were power available to do so; two MPPT controllers on one PV array is a recipe for disaster. Don't do it.

Shall I even mention that running a 90 Vmp array for a 12 Volt system is a mistake too?

I hate to say it but as described this system has "done wrong" written all over it, and attempting to add a tap for the 24 Volt starting batteries would only make things worse. If you absolutely need to and have an inverter running on the 12 Volt system use it to power a 24 Volt battery charger. Otherwise ... more panels are needed.

Photowhit · October 2014

Re: Add starting bank charging to solar system

While acknowledging the odd system, I suspect you use a generator for your primary charge source?

....Your starting batteries shouldn't loose much charge at all, and should be fine left for several months! If they are discharging, perhaps you have some loads on them that should be on your boon docking system? Maybe a $20 battery disconnect would serve you better?

barclaybrown · October 2014

Re: Add starting bank charging to solar system

Odd system? No idea what you guys are talking about. I know I shouldn't question free help, but I'm not understanding your point. Yes, of course there is other equipment like Trace inverter, 14kw genset, etc. I'll guess that you guys are more used to home systems that have lots of PV wattage and don't need much battery capacity at all. I don't know of any vehicles that have room for a 2.3kw array!

But anyway... yes, theoretically starter batteries should not run down. I suspect the coach has some loads and the two cut off switches in the engine bay don't fully disconnect the bank. There are also equalizers on that bank for coach 12v loads like dash instruments and headlights. I may try further tracing to find the loads. But I also think it's better to have a maintaining charger on the house bank just in case. This is not that unusual. And yes, my solar often has power to spare once the house batteries are charged up, or if I'm on shore power.

So having just exhibited the bad manners to bite the hand that's trying to feed me, I'll ask again. Can I connect two charge controllers to one solar array? I had thought this was possible.

I don't see how I can use a 12 v to 24 v charger because it would be connected to the 12 v input/output of the inverter and thus would draw down the house bank to charge the starting batteries, and I don't want to do that. More practical and obvious is a 110 VAC charger than runs off the inverter, and thus indirectly uses the solar that is charging the bank that powers the inverter. But I'd have to control this manually, since I might not want it to draw down the house bank either. That's why I'm thinking to charge the starting bank only off "spare" solar power stolen from the house bank before the house bank even sees it!

I'm guessing that with two controllers on the solar array, the starter batteries would charge up fast since they alternator probably brought them almost all the way back up while driving, and then would draw little more as the solar power went to the house bank. And yes, I can also do math. If I discharge a 1500 AH bank 50% it would take 750 watts 10 hours to recharge, even more with normal yield and losses--likely 20 hours or even more if there are active loads drawing while recharging. It is unlikely I can run solely on solar if dry camped, especially with a full size house fridge, big computer, 50" TV with surround, etc. etc. The solar is a supplement to the other means of charging (genset, shore and engine).

Thanks for any further input!

Cariboocoot · October 2014

Re: Add starting bank charging to solar system

As a rule no, you can not connect two MPPT type charge controllers to one PV array. I thought I already explained that.

What's unusual about your system is three things:
1). Huge 12 Volt battery bank. 9kW hours DC stored power is really big. Bigger than most off-grid stationary systems.
2). Very undersized array for same. It could do very little towards actually charging the batteries if they are drawn down. If it seems to be working beware that it may just be deficit charging that will eventually catch up with that huge battery bank.
3). Use of 12 VDC 'house' bank when 24 VDC is used for engine. If both were at the same level it would be a lot more efficient and simpler to interconnect the two if needed.

You don't have any 'surplus' PV power to work with, so it matters not that running an AC charger from the house bank & inverter is less efficient than directly from the PV.

Battery charging isn't linear nor a matter of applying 'X' Watts for 'Y' hours until it's recharged. If minimum current isn't met the recharging doesn't happen. At 3% current with all loads off you could charge all day (5 hours) and lose half to self-discharge overnight. It could take you a week of no loads to bring that battery up from 50% from the solar alone.

barclaybrown · October 2014

Re: Add starting bank charging to solar system

What about one MPPT type solar controller like I have plus one non-MPPT controller?

Where do you get 3%? 3% of what? In full sun, this solar array puts out, as you said about 48 amps. How do you say that this is insignificant when it comes to charging? It's nearly half of the normal charge current I get out of my big inverter/charger!

For anyone else reading, this size battery bank is completely normal in high end coaches. Yes, if you draw it down to 50% and have only the solar to charge it up, it will take a long time. If you draw it down say 10%, then you have to run the genset a lot less (or none at all) to charge it back up the next day, if you are not drawing much during the day. It's not like a house. So, hmm maybe I am better off just powering a 110 VAC charger off of the inverter. If it's a good one it won't draw much when the batteries are charged, and the starting batteries won't normally be deeply discharged at all.

But I sympathize. I'm always trying to engineer things a new way and do it better than "normal". I have a normal 150 amp charger on the house bank and it doesn't charge fast enough for me! With a 14kw genset I should be able to charge faster by breaking the bank apart and using multiple chargers. I believe it will work and the experts (e.g. PhD battery engineers) are divided about whether it will work. It's a science experiment! I have the relays to separate the bank, but I haven't yet bought the extra chargers.

Photowhit · October 2014

Re: Add starting bank charging to solar system

barclaybrown wrote: »

For anyone else reading, this size battery bank is completely normal in high end coaches...

Well I find it a bit unusual, just so we're comparing apples to apples, 1500 Amp Hours at 12 volts would be 14 - 215 Amp hour 6 volt golf cart batteries. My home system uses nearly the equivalent in a 24 volt system 800 Amp hour 24 volt battery! My battery weighs a bit less than my econo-box car! I have a friend who travels with Beaver Motorhomes, they ain't cheap and he often switches out the 4 battery reserve for 6 at 660 amp hours. I don't consider it 'normal'.

I think a separate system, might serve you best, or a battery maintainer off the large bank. Since there shouldn't be any loads on the starting batteries this should work fine, starting batteries aren't designed to be deeply cycled so I don't think you will need much current coming in.

Cariboocoot · October 2014

Re: Add starting bank charging to solar system

It is possible an MPPT and a PWM controller could share a common array, but it is still a gamble as you don't know how paralleling the inputs would affect the operation of the MPPT (they basically are not designed to have something other than PV across the input, especially not an active circuit). I'd ask boB about the advisability of this.

48 Amps is about 3% of 1500 Amp hours, so in respect to how much battery it has to charge it's pretty insignificant current. Battery manufacturers tend to recommend 5% minimum (of capacity in Amp hours at the 20 hour rate) which in this case would be (1500 * 0.05) 75 Amps. In general we go for 10% peak rate, or 150 Amps for such a large battery bank.

I'm not surprised they put such a bank in motorcoaches; in my experience the people who design the electrical systems in automotive applications tend to do things rather differently than what most engineers would recommend. As Photowhit points out, such a bank could be a real nightmare of numerous cells and wires and represent quite a challenge in current handling. It is also doubtful that it is necessary, but I'm sure it's expensive and prone to running up lots of repair charges which is of course what really counts, right? Bloody awful design to my way of thinking.

The big question I'd be asking is: are all those Amp hours necessary? You'd be amazed at how many systems I've fixed by the removal of batteries. I'll bet the people who put that together fell into the old "lots of days of autonomy and a little PV will charge it eventually" trap.

Photowhit · October 2014

Re: Add starting bank charging to solar system

Here's a Battery maintainer that should keep your starting batteries charged up, just plug it into your 24v 'house' system and since it's monitored and charged all should be well.

NOCO Genius G26000

I think this would be easier than having 2 solar charging systems or trying to switch a single charge controller back and forth...

PS-This does run off the inverter, I put in the 24v to indicate I wasn't speaking of your 'home with a yard' system, I don't think you would have a problem, but I don't speak 'Motor Coach' very well and I'm sure it might confuse others.

zoneblue · November 2014

Re: Add starting bank charging to solar system

Bob has repeatedly said that you should not connect anything else to the input side of a classic. Where people have persisted he has insisted on back feed diodes. The kid on the other hand does have a special bully mode feature that specifically allows this.

However i think there are lots of better alternatives for what you are trying to do. A cheap smart charger/battery maintainer, plugged into the inverter. Or a dc-dc boost converter could be put together quite cheaply to provide a trickle, current and voltage limited, direct from the house bank.

Another odd thing in this system if you ask me is that its 12v at all. I bet the inverter is 2kW or bigger, and draws hundreds of amps.

If i were you id be listening to the guys, they know what they are talking about. Just a few years ago when pv was expensive and batterys were cheap systems like yours were more common. Nowadays thats all changed and we are much better able to care for our banks with a proper ratio of solar to bank. You want a 0.1C charge rate, thats the goal.

And... FTR, there are several multi kW RV installs, right here on this forum. PV are getting more efficient and take up less space than they used to. All the "high end" motorhomes ive ever seen have lots of roof estate. It might be cluttered with stuff scattered all over, that never considered the future trend with solar, but that isnt our fault.

barclaybrown · November 2014

Re: Add starting bank charging to solar system

Because I'm asking a question and am new on the forum, am I automatically in the "don't know anything" category while the guys who have a long history on the forum are assumed to "know everything"? I guess beggars can't be choosers. Several people have puzzled over why I have a 12v house system and a 24v starter system, which indicates a lack of experience with RV systems, at least those in high end coaches. The coach came with a 24 volt starter system, so that's that. The house system CAN be designed to use 24 volts, but very often they are not. My previous coach used a 4kw 24v inverter, and then had three large voltage reducers to provide 12 vdc to the various systems in the coach that required that, and yes, the two banks were tied together when the bus engine was running. Someone suggests that my battery bank is too big for a coach. Nope, mine is not at all the biggest out there. Larger banks allow more use before recharging, obviously. The fact that the solar array can't recharge the entire bank quickly would be a silly reason to reduce the size of the bank. There are, as someone else mentioned, other charging sources, including shore power, a separate bus engine-driven 12v alternator, and a 14kw genset.

As for larger solar arrays on a bus, well, the latest Kyocera panel is 320 watts and is some 55x65" call it 4' x 5' to round off. How many of those fit on a 8' x 40 or even 45' roof less all the other equipment up there? Absolute max would be 10 of them, so yes, I suppose it's possible to get "multi-kilowatt" under ideal circumstances. But all RVs have at least vents up there and most have air conditioners, satellite dishes, antennas, etc. I'd like to see how someone gets more than 2kw. I don't think I can fit more than perhaps one more panel giving me perhaps 1kw and I don't even have roof air conditioners up there. But I grant you it is theoretically possible to get over 2kw. But rarely practical.

I have no idea where those rules of thumb about the relationship between a solar array's power and the size of a battery bank came from, but clearly they have nothing to do with how an RV uses a house battery bank or solar array that I can see. Calculating a percentage from a number of amps and a number of amp-hours makes the engineering professor part of me go crazy!

The rule in RVs, and of course this only applies to the few people who are designing their own house systems, is have as big a house battery bank as you can afford the cost and space for, and have the most solar you can afford as well. Then have the biggest battery charger you can afford too to charge it as fast as you can from your genset or shore power. Oh, and still have the most efficient appliances you can so you use as little power as possible. That's why my 50" TV is now an LED type.

I hope this is a little informative rather than just a rant from a newbie who doesn't know anything

Oh, and those who did respond directly to my question were helpful, and yes, I think it best now to use an AC-powered charger connected to the inverter (I already have one connected in fact) and live with the possible disadvantage that if there is some kind of drain on the starting batteries, it would also draw down the house batteries if the solar couldn't keep up with the overall need--not a big worry unless something goes really wrong, and in fact, this approach adds the additional safeguard of the low battery-cut off feature of the inverter.

Oh, incidentally, the Noco chargers get some scathingly bad reviews from some engineers who measured and tracked their behavior during charging cycles. I have no way to judge the validity of their experiments, but read the reviews on Amazon for some of the details. I have used a Battery Minder and have had very bad experiences with those as well. One just quit, which allowed my bank to discharge completely once, and the replacement they sent, seems to just stop charging sometimes and flash an error light and thus let the bank go down to nothing over time... not what you want in something to maintain batteries over a long period of storage!

Thanks for the conversation--always fun talking about these things.

BB

Photowhit · November 2014

Re: Add starting bank charging to solar system

Likely most of what we discuss here are Cabin and home based system, RV systems are a bit different, the close proximity makes 12v applications more viable, since you would not need to run massive wires to prevent voltage drop. Perhaps as an engineer, you would understand the advantages of having minimal amount of battery strings in a system, since charged from a single source once the resistance gets out of balance, and it will the charging will become different between the batteries. In such a system we do recommend that the size and gauge of wires is the same to help maintain the same resistance for each string.

The rules of thumb for battery charging don't come from us but from the battery manufactures, for flooded batteries 5-13 % is standard. I actually went and checked your previous posts, to see if you had incorrectly determined your battery bank, perhaps 6 - 230 Ah 6 volt batteries, and not figuring the amperage correctly, but indeed your have 6 - 250ah AGM batteries(from previous posts), They may have different recommended charging rates.

I don't know if there are better charger maintainers, I pointed to the Noco only as an example. We did use on at the battery shop and used it as the preferred charger, but I have no personal experience with them other than that.

I do have real world experience with RV, and have seen may travelers, I use to live in a cabin and work security in a 2600 acre camping community. Since I live on solar electric, I was the person most often called when people had problems, your battery bank is larger than any I have seen, I'm sure people have larger. Roy Mueller of the Beaver Club is a friend of mine and I'll ask him the next time he passes through, as I said he has swapped out some 4 6v GC battery systems for 6 - 6v systems, and we had an outage worker who had 4 big 12V AGM's they might have been 250 ah run as a 24v system, I was amazed by that large of a system...

FWIW- if you are looking for the most energy efficient, Sun Power typically has the highest energy per sq inch, It has a unique design with the wiring on the back not shading the cells and can squeeze out a couple percent higher than most other manufacturers. They are hard to come by as they like to install their own product, but if you work at it you can find them.

Have many fine travels, enjoy the scenery1

Iceni John · November 2014

Re: Add starting bank charging to solar system

Barclaybrown,

FYI, I'm converting my Crown bus into an RV now, and today I just finished the wiring for the PV panels to their combiner boxes. I have eight Sharp 255W panels in two rows, hinged off a central walkway so they can face the sun, and they will feed two 60A Morningstar MPPT charge controllers for eight golfcart batteries giving me about 900Ah at 12V - the charge rate is about 13%, so that should fully charge them by noon or early afternoon. One may be able to have more than 2kW of PV on a S&S RV, especially a 45', but it would compete for precious unshaded roof space with the roof warts etc. I too am deciding if I can also run some small PWM controllers off the PV arrays to keep the starting batteries charged, or if some other plan is better - some folk here think I should run the SLI chargers off the inverter instead.

It sounds like you and I are encountering some similar problems, but my solutions will probably be much different than yours simply because I'm building my bus for generator-free long-term off-grid boondocking, not exactly what most RVs are built or used for!

John

barclaybrown · November 2014

Re: Add starting bank charging to solar system

Wow, very cool project there! I assume your coach is not what we call "all electric". An all electric coach has a home type or at least electric-only refrigerator, and usually has no propane system. Some have a hyndronic aqua-hot system for heat and hot water, but everything else is electric. The fridge is my main problem in living without the genset. I want to replace the fridge with a newer more efficient one, but that's a big job since it has to come and go through the windshield! Sounds like you have the solar system well in hand for your coach, but depending on what you do in the coach, how much power you like to use, and how much time of the day you'll be in the coach, you might find the battery bank a little small. Your solar has a theoretical output of what, 170 amps total, but what are you counting as realistic. Others will know better than I about that. And of course you don't want to discharge your batteries more than 50%. My question would be, in the worst case, will you have enough battery capacity to make it from sundown to sunrise? And then how much power will you use WHILE the solar is trying to recharge the batteries? If I were you, I would just plan some additional space, if possible for additional batteries if you want to add them later. And yes, my conclusion is to use an AC-powered charger for the starting batteries. If your starting system is 12v though, you could throw the starting batteries in parallel with the house bank, but only when you have good charging (bright sun) going on. And I also hope you're planning to have a genset just in case of cloudy days and other random conditions. It could even be a small one, like a honda to have just in case. Of course you'd need a genset if you ever plan to use air conditioning!

If you think about it the only downside to have more batteries is cost and weight. They don't affect how much power you use, and discharging a bank by 25% is better for it than discharging to 50%.

Awesome on the self-conversion--I've never done a complete one... just reworked aspects of my old 72 GM 4108 when I had it and now the 93 Prevost.

BB

zoneblue · November 2014

Re: Add starting bank charging to solar system

barclaybrown wrote: »

Because I'm asking a question and am new on the forum, am I automatically in the "don't know anything" category while the guys who have a long history on the forum are assumed to "know everything"? I guess beggars can't be choosers.

All i can say is that if you ask for advice it pays to listen to it...

Several people have puzzled over why I have a 12v house system and a 24v starter system, which indicates a lack of experience with RV systems, at least those in high end coaches.

12v systems are only a tradition, nothing more. 24v fridges, pumps and every single other RV applicance are available. For a bigger system, such as you are talking about, a higher system voltage reduces cabling costs and reduces losses everywhere. It also reduces the size of some system components for example charge controllers can do double the watts throughput at double the voltage.

For the inverter a 4kW inverter running on 12V, is 4000W/11V dropout /0.85 efficiency = 430A. Surge, double that. Thats a lot of amps.

Someone suggests that my battery bank is too big for a coach. Nope, mine is not at all the biggest out there.

Again, their existance doesnt make it right.

Larger banks allow more use before recharging, obviously. The fact that the solar array can't recharge the entire bank quickly would be a silly reason to reduce the size of the bank. There are, as someone else mentioned, other charging sources, including shore power, a separate bus engine-driven 12v alternator, and a 14kw genset.

If you have all these other charging sources, then why do you need the big bank? Lead acid is not like lithium where it can sit at partial SOC for long periods.

As for larger solar arrays on a bus, well, the.......snip

Sorry we challenged your world, but this is how we see it, in our world.

barclaybrown · November 2014

Re: Add starting bank charging to solar system

>> All i can say is that if you ask for advice it pays to listen to it...

Thank you for your advice.

ZoNiE · November 2014

Re: Add starting bank charging to solar system

Perhaps we need an RV section on the forum. I am an RV Solar user too, just starting to play with my 390W system on my Travel Trailer. I am learning all I can so when I build my bus in the future, I'll be ready.

As for the reason the larger high end rigs use such large battery banks is really simple. Mamma wants to run the microwave and the coffeepot in the morning while watching the morning news without starting the genset or the the coach. That current has to come from somewhere. the 12-14KW gensets usually are what take care of this recharging.

I make my coffee on the stove...

rplarry · November 2014

Re: Add starting bank charging to solar system

Barclaybrown
Can you describe your 12v bank to me? How many batteries and what voltage and AH rating? I might have an idea for you.
Larry

barclaybrown · November 2014

Re: Add starting bank charging to solar system

rplarry wrote: »

Barclaybrown
Can you describe your 12v bank to me? How many batteries and what voltage and AH rating? I might have an idea for you.
Larry

Sure. Bank is six 12-volt AGM 8D size batteries, 250AH each, so 1500AH total. They're in parallel.

I plan to do some testing in Florida on living off solar only. It will really depend on how many hours a day/night I'm in the coach using the computer and audio/video system, each draw perhaps 200 watts or so. The fridge, now 21 years old, is the big hog. I'm wondering how much energy I'd actually save if I replaced it with a modern one.

Question for you... how often do you run your genset, and how big a battery charger do you have to use with it? I have been investigating ways to charge my bank faster using my 14kw genset. I have a 150 amp charger (Trace innvter/charger SW2512) and of course it uses only fraction of the 14kw abilities of the genset. It seems wasteful, and is certainly noisy and smelly to run the genset longer than needed. My general idea is to split the bank up into 3 pairs of batteries (using some big relays), and charge each with a separate charger, for say two hours, then recombine to finish up the charging, or shut off the genset and let the solar top it off.

Cariboocoot · November 2014

Re: Add starting bank charging to solar system

barclaybrown wrote: »

Sure. Bank is six 12-volt AGM 8D size batteries, 250AH each, so 1500AH total. They're in parallel.

Well there's a problem right there: good luck getting even current on six parallel batteries, especially at 12 Volts.

If I were you I'd seriously reassess the size of that battery bank. You may actually get better performance from a smaller bank. 1500 Amp hours @ 12 Volts? What do you need 9kW hours of stored power for?

Unfortunately since they are AGM you can't even evaluate their condition.

Plug that refrigerator into a Kill-A-Watt and see how bad it is. If it uses more than 1kW hour per day you may indeed benefit from a replacement.

barclaybrown · November 2014

Re: Add starting bank charging to solar system

Cariboocoot wrote: »

Well there's a problem right there: good luck getting even current on six parallel batteries, especially at 12 Volts.

If I were you I'd seriously reassess the size of that battery bank. You may actually get better performance from a smaller bank. 1500 Amp hours @ 12 Volts? What do you need 9kW hours of stored power for?

Unfortunately since they are AGM you can't even evaluate their condition.

Plug that refrigerator into a Kill-A-Watt and see how bad it is. If it uses more than 1kW hour per day you may indeed benefit from a replacement.

Thanks for the kill-a-watt idea... hadn't thought of that, but it's a great idea because the fridge cycles on and off. Other things I can easily measure with current monitors.

Why uneven current? Are you assuming a voltage drop across the 18" or so of 4/0 cable connecting the batteries? Within a very small range, I would think that the + and - terminals of all six batteries would be identical in potential. I've seen this claim made before and the electrical engineer in me just doesn't understand. If the thought is that some of the current "diverts" into the first few batteries, so less makes it down to the other batteries, this seems to reflect a misunderstanding of Ohm's law and how basic circuits work.

As for the bank size, RV systems are different, as we discussed below.

Cariboocoot · November 2014

Re: Add starting bank charging to solar system

Uneven current because it is impossible for the resistance to be the same on six batteries in parallel.
Even using large wire like 4/0 and equal lengths all going to bus bars does not eliminate the difference in battery resistance. As they get older, they will change more. Resistance will increase unevenly, and not just for the batteries but the connections as well. AGM's have inherently lower resistance than flooded cells so the problem will not be as noticeable, particularly with newer batteries.

On the whole if you need large capacity your better off with higher Amp hour cells in series rather than multiple parallel connections. Unfortunately this is not always possible to achieve, especially in a mobile application where there are space constraints.

RV's are not really different from any other off-grid installation: power use is power use, stored capacity is stored capacity, recharging is recharging. The biggest difference is that the people who design automotive/RV electrical systems all seem to have been hit in the head repeatedly with an iron bar until they are no longer capable of sensible thought. I've seen lots of them, and they are convoluted designs to say the least.

barclaybrown · November 2014

Re: Add starting bank charging to solar system

Cariboocoot wrote: »

The biggest difference is that the people who design automotive/RV electrical systems all seem to have been hit in the head repeatedly with an iron bar until they are no longer capable of sensible thought. I've seen lots of them, and they are convoluted designs to say the least.

No point in arguing here, and I intend only to explain my system as it's relevant to the discussion, not to defend it. Your point of view, is, by your own admission, different than the bulk of the industry in mobile systems on RVs, coaches, yachts, etc. People can make their own decisions. I just didn't want to let your views go unchallenged in case someone might think they are the last word.

zoneblue · November 2014

Re: Add starting bank charging to solar system

barclaybrown wrote: »

Sure. Bank is six 12-volt AGM 8D size batteries, 250AH each, so 1500AH total. They're in parallel.

Far, far from ideal. Other than issues with eneven wear and tear, you have a real possibility of a major battery fire, should one of the batterys short. A simple manufacturing defect, or a chunk of plate shedded, can dump 10000 amps from the other batterys into the failed one, then the whole mess goes into thermal runway. If you do do parallel, even though its a non recomended setup, you need fusing on each string. They either need to be ignition proof (with very high AIC) or greater than 500mm from the bank. Secondly if you have laddered the batterys, thats wrong, you must use a bus system for that many batterys in parallel. IF you must have a 1500Ah battery then best practice is 2v cells in series.

It will really depend on how many hours a day/night I'm in the coach using the computer and audio/video system, each draw perhaps 200 watts or so. The fridge, now 21 years old, is the big hog. I'm wondering how much energy I'd actually save if I replaced it with a modern one.

Dont wonder, you need to understand your demand Wh/day. People do this thing, they spend 10s of K on energy gear then blow it away on old energy hog appliances. Energy conservation is the first rule of RE/off grid. A poorly performing fridge is a MAJOR energy drain. Computers may be rated at 200W but few actually draw that. And if it DOES draw that, (hell even if its rated at that) then you need a new one. Power is much much easier to save than generate, especially in space constrained mobile applications.

Your point of view, is, by your own admission, different than the bulk of the industry in mobile systems on RVs, coaches, yachts, etc. People can make their own decisions. I just didn't want to let your views go unchallenged in case someone might think they are the last word.

Nah mate, i think you will find that it this coach world of yours that is at odds with best practice in the industry .

barclaybrown · November 2014

Re: Add starting bank charging to solar system

Thanks guys for your input. Fuses are a good idea.

Other than that, I'm done here. Please feel free to close or delete this entire thread, since obviously your view is that my set up is dangerous. I wouldn't want someone else being misled by what I've done. Ciao.

Cariboocoot · November 2014

Re: Add starting bank charging to solar system

barclaybrown wrote: »

Thanks guys for your input. Fuses are a good idea.

Other than that, I'm done here. Please feel free to close or delete this entire thread, since obviously your view is that my set up is dangerous. I wouldn't want someone else being misled by what I've done. Ciao.

No one said it was dangerous. It's just not efficient.

Unless you know what your load needs are you can not size a battery bank properly.
If you can do what you need to do on less battery, you instantly come out ahead; less gen run time, better use of solar, more efficient over-all, lower replacement costs.

If the coach world knew what they were doing they would have put a 24 Volt house system in to match the 24 Volt engine system. It would then be a simple matter to 'borrow' power between the two if it were necessary.

Let me remind you that it was you who wanted to be able to charge the 24 Volt engine system from the solar, which is entirely sensible. But not so easy to do when the house system operates at half the Voltage.

Keep in mind that I personally have fixed dozens of systems that did not work right, many of them designed and installed by people who claimed to know what they were doing. That would include some amazing professionals of the RV world who sell people whatever they've got because they either don't care or don't know any better.

Photowhit · November 2014

Re: Add starting bank charging to solar system

barclaybrown wrote: »

Why uneven current? Are you assuming a voltage drop across the 18" or so of 4/0 cable connecting the batteries? Within a very small range, I would think that the + and - terminals of all six batteries would be identical in potential. I've seen this claim made before and the electrical engineer in me just doesn't understand. If the thought is that some of the current "diverts" into the first few batteries, so less makes it down to the other batteries, this seems to reflect a misunderstanding of Ohm's law and how basic circuits work.

Before you shrug us off, I wanted to give you this link to a site on the issue of parallel strings, in hopes that it will prolong the life of your batteries!;

Among the comments you will find are "The problem is that in very low resistance circuits (as we have here) huge differences in current can be produced by tiny variations in battery voltage" While he has done some calculations, I have also seen a measured current showing a 15%-20% difference in the current flowing from the front set of a 'laddered' string of batteries and the 3rd(or maybe 4th) set.

It's not very expensive and if your considering adding fuses/breakers, it might be a good time to switch the wiring to prolong the life of your batteries.

Also when it does come to the time when you must replace your batteries, and if you have the desire to get the most charging from your solar, you might consider going to a LiFePo battery bank, perhaps 1/2 the weight for a similar amount of storage, a bit expensive now, but prices should be coming down as they gain greater market share. While expensive now for off grid use, they have the advantages of being smaller and lighter than lead acid and have a higher charging efficiency (80-85% vs @95%). We have had several threads about them lately and they have real advantages for RV use! Not so much (yet) for off grid.

scrubjaysnest · November 2014

Re: Add starting bank charging to solar system

Ran into a set-up very similar to the OP's. We lost shore power and I was asked by a fellow camper why his genset couldn't keep the batteries up. His set up was 6 of the 12 volt trojan 8D's. When we lifted the access door so I could check loads etc, you could here the inverter screaming. The clamp on showed it drawing 67 amps. Had him check what was running inside. Forcing the refrig to propane dropped the inverter draw 59 amps. Turned out the refer outlet was configured to switch to inverter on loss of shore power. The TV going in the bedroom with no one watching it was several more amps. The porch light on in the day time was another 1.5 amps. As a general rule the refrig should switch to propane on loss of shore power; not all RV's have two or three way refrigs. Most RV dealers don't have a clue how to install solar systems. My system is under paneled even for our light usage; something I've known and currently taking steps to fix as time allows. Good unbiased reading is batteryfaqs.org and smartgauge.co.uk/batt_con.html. For amusement read HandyBob's blog. There are others out there on the net and they all agree with what has been pointed out in this thread.

Add starting bank charging to solar system

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