Money and No Brains

fowlerrudi · October 2012

Hello,
First post, long time lurker.... man it seems like there are a lot of me on this site!

I have been off grid on a 2000eu Honda for 4 years now. Tried a 3000eu for a bit too.

Just built a brand new house, 3km from the nearest power line. Log home, all in floor heat - wood boiler and alpha circ pumps. No elements in this home, all CFL and LED lighting.

Just installed/installing the following:

Xantrex 6048 Inverter
Xantrex 80A MPPT 600V Charge controller
4600 watts of Conergy Panels (20 panels at 230w each)
24-85-25/ 48 volt / 1608ah @ 20hr rate / 4000 lb Deka D-Series forklift battery

The questions:
Will I need a generator to EQ this battery?
How much generator do I need to EQ this bad boy?
Do I have enough array?

I just hooked up a 4000 watt generator to charge this battery up until we hook up the panels this week. I used some Deka parameters for forklift batteries that I got from this forum. I don't think I can properly charge this battery without the array since the generator seems to be junk and kicks off when putting out any more than 1600 watts. Anyway, I have been running some loads off the battery - shop vac, router, saws etc. Battery was at 55.5V and showing fully charged, after a night of working at the house, it was down to 49V. Seems like it is dropping some fast. There is a ton of KWhr in this bad boy to be dropping that fast. If I fire up genny again it will top itself back up to 55v I believe in under an hour. Seems odd. Maybe I shouldn't worry till I can get the array hooked up and get some serious juice flowing. I am however concerned that I have too much battery. I have enough knowledge to be dangerous. Thanks for your help!

Cariboocoot · October 2012

Re: Money and No Brains

Welcome to the forum.

The XW 6048 has a charge current limit of 100 Amps, which is a tad low for 1608 Amp hours of battery. At full power it would suck that 4 kW generator completely: 100 Amps * 48 Volts = 4.8 kW, and that's not the end of it.

Will the panels do it? Nope. Again you fall short. If you were trying for a 10% peak charge current on that battery bank that would be 160 Amps @ 48 Volts and would require about a 10kW array and two 80 Amp charge controllers. Why on Earth did you buy that 600 Volt Xantrex unit? A lot of money for an inadequate controller. Could have got two MidNite Classics for that.

Why did you go with that whopping big battery? That represents about 38 kW of stored power!

westbranch · October 2012

Re: Money and No Brains

that battery, using round figures has 77 KwH of total power and about 38.5 Kwh of power available... no wonder that 4 KwH gen set has a problem...

First what are your loads? this is the starting point of system design and , being specific, what are they? There is a great difference between motors and resistive (lights) loads...

What are the specs on that gen set?

more later...

fowlerrudi · October 2012

Re: Money and No Brains

Big whopping battery justification - refurbished forklift battery - the price was right.

600V Xantrex justification - Wanted to interface with the 6048 and wanted 600V. I can only support 4600 watts on this unit and will have to add a second smaller charge controller I believe in order to stop the controller from 'clipping.'

The generator is a 4000 watt Onan that I robbed from an uncle's cranberry farm for the evening - since my Honda EU3000 doesn't have 240 and the 6048 only accepts a 240 gen input.

I anticipate 10KW per day load, however have not moved in yet so I have no idea - its all theory at this point. Currently I use 1.5 kw/hr a day but this home will be a bit more power hungry due to the heating system, etc.

Forklift batteries are meant to do 80% discharge, I was hoping to have a larger battery to float more snowy and rainy days.

westbranch · October 2012

Re: Money and No Brains

Sadly, I would say you are a victim of "ready,....FIRE....aim"....

So the question now is, what are you prepared to change? cause what you have now ain't going to work, sorry.

ps 77Kwh * 80% = ~60Kwh @ 4 Kwh = 15 hours.... see what is happening? You want to recharge within say 4 - 5 hours as that is the average amount of good sunlight you might get depending on location.

Cariboocoot · October 2012

Re: Money and No Brains

The problem is coming up with enough charge power to bring that monster battery back up.

Minimal rate of 80 Amps is still a lot of power need: about 5kW worth, so you're still short of charge capacity.

Well you've got what you've got, but it's not going to be great. Definitely need to get a big capacity generator to make up the shortfall. At that the XW will only make 6% from the gen.

Equalization isn't going to be the problem (higher Voltage but not much current), Bulk charging is.

fowlerrudi · October 2012

Re: Money and No Brains

That's why I'm posting, sock it to me. Smaller battery or bigger array? I have get to buy a genset, nor do I want to. Will I need to in order to EQ?

westbranch · October 2012

Re: Money and No Brains

OK just what do you have and what is yet to purchase?

fowlerrudi · October 2012

Re: Money and No Brains

But what if my battery only gets drawn down 10%? If I take it down 80% then I will need multiple days to charge it back up. A good day here in NB is putting 30kwhr into batteries with a 5kw array - based on a neighboring system.

fowlerrudi · October 2012

Re: Money and No Brains

I have the XW6048, the 80A/600V matching XW charger controller, and bertha - the 1608AH 48V king of the forklift world. I have a 20 panel array of 230w conergys. I have room for 5 more panels on the mount but will need a second charge controller to take advantage of it.

westbranch wrote: »

OK just what do you have and what is yet to purchase?

westbranch · October 2012

Re: Money and No Brains

An easy trap to fall into is to overestimate your input and underestimate your use.

Better to plan on 4 hrs of good sunlight ...on AVERAGE... and use 'opportunity loads ' when the power is available.

10% is still 160A * 48 V = 7.7 KW

Cariboocoot · October 2012

Re: Money and No Brains

fowlerrudi wrote: »

But what if my battery only gets drawn down 10%? If I take it down 80% then I will need multiple days to charge it back up. A good day here in NB is putting 30kwhr into batteries with a 5kw array - based on a neighboring system.

If you're only going to take it down 10%, you don't need such a big battery. Believe it or not, cycling a deep cycle is good. Not cycling it, not so good. And if you're counting on all that capacity for reserve (cloudy days) you still need a way to push it up quickly. Sometimes a bargain is not a bargain.

Were it me, I'd scale back the battery capacity to something that better fit the power demand. A bit hard to do after already having bought that bank.

fowlerrudi · October 2012

Re: Money and No Brains

I'm really appreciating the help guys. Can you educated me on this 10% rule of thumb you are using? In laymen's terms? I also have a 7.5 KW diesel genset, but it's ugly and old. Was hoping to be able to use the eu3000 with a transformer.

I just built a shed around that battery. It weighs 4000 lbs. I would rather add panels than replace it.

westbranch wrote: »

An easy trap to fall into is to overestimate your input and underestimate your use.

Better to plan on 4 hrs of good sunlight ...on AVERAGE... and use 'opportunity loads ' when the power is available.

10% is still 160A * 48 V = 7.7 KW

westbranch · October 2012

Re: Money and No Brains

well it is not a 10% rule, we try to plan the capacity of a system to last for 3 days , to account for an extended period of no sun, ie winter storm. It is good practice not to go lower than 50% DoD to ensure you do not damage your battery. The recharge capabilities (PV and Gen) should be able to recover the battery to > 90% SOC in one day. You will have to do the math to verify just what your PV can do. Remember the rule is PV rating * 50% for what you can get out of your battery.... Post it and we can proof it for you.

There are Honda inverter gen sets in the 6 - 7000W size that might work for you but with little to spare if any.

Idea: can you return you CC and inverter and exchange them for a 24 v version (4024)?... as it seems you are settled on that 1600A battery. At 24 volt config (1/2 the existing power) you might be able to make things work...

HTH

add: 10% of the battery capacity is still a huge chunk of power!!

Ken Marsh · October 2012

Re: Money and No Brains

Do not despair. You did better than most people on their first solar try, including me.

Equalization is no biggie. With new batteries you shouldn't have any problem.
Just get them up in the high 50s volts now and then.
You can tell by listening when they start to boil, usually about 56 volts.
The power is in the bulk charging.
If that big 6048 kicks out the generator you can program the charge rate back.

Theoretically your charge controller does not have the capacity to quite fully utilize your PV.
But that is not a problem either. They self limit.
Do not know where you are but if it is anything like Indiana, the PV is way short of SEC capacity during winter months.

Battery voltage drops rapidly from 55 volts, particularly with larger loads like power tools.
It is good if you can keep the voltage up there but most battery capacity comes at 50 volts and down.

Sounds interesting, keep us informed.

Ken Marsh · October 2012

Re: Money and No Brains

Sorry about the late post. I got distracted and by the time I posted there were 13 posts.

Anyway, I hate to be a non-conformist here but I feel you guys are being over critical of this install.
Two years ago I helped with a installation in NE Washington state that has 4.5 KW fixed PV with 66 KWH of HUP battery.
This is a similar battery to fowlerrui's.
They routinely discharge to 80% DOD and they only charge with the PV.
It works great. No problem charging or doing equalization.
They rarely do equalizations anyway.
The bigger battery really makes it nice because of its carry over ability.
And this type battery is different from starter batteries and their deep cycle derivatives.

Eric L · October 2012

Re: Money and No Brains

It might be worth mentioning that, while the recommendation is for the battery bank to receive at least 10% of the 20-hour rate amp-hour capacity in charging amps, this isn't something I've seen anyone claim needs to be provided every day. It may be that once a week or less at this rate of charge would be sufficient (I'm guessing with this once a week number here, but the OP could start with this kind of charge routine and see how his battery sg did over time).

So as suggested by posters above, a 6 KW generator into the inverter, combined with the 60 or so charging amps from the pv, would get you to about 160 charging amps, or 10% at around 57 volts charging. So when the sun is shining on the panels, you'd then also be firing up the gen from time to time to supplement the charge in order to get a good bulk charge on the batteries. Maybe make sure the batteries were more deeply discharged from time to time ahead of these supplemented bulk charges, assuming they aren't so through daily use.

What will seem strange is firing up the generator on sunny days, especially if your day-to-day use isn't enough to draw down the batteries sufficiently for the generator-assisted bulk chargings, which you'd still need to provide for battery health.

Cariboocoot · October 2012

Re: Money and No Brains

Right. Better clarify the 10% notion. Again.

First of all, it is a potential peak charging current, not the current you will see or must have to charge the battery.
Second, it is a net charge rate meaning what you get minus any load draw that occurs while charging (like running the inverter).
Third, the main reason you need it is to support the charging after those three dull days when the battery has been dragged down to 50% SOC and needs to be brought up quickly.

For a well-balanced system, this 10% rule-of-thumb works nicely with a 25% DOD and two days reserve power (day three; start the generator).

It does not mean a system using a lower charge rate will not work (although going below 5% is almost certain disaster).

But even to achieve the minimum 5% rate you need a lot of panel on that battery: 4987 Watts and one 'maxed out' 80 Amp charge controller. In my opinion the performance of such a system would be disappointing to say the least.

As it is, this system is not well-balanced and will not perform well.

Feel free to disagree; it's not my money you're spending.

stephendv · October 2012

Re: Money and No Brains

fowlerrudi wrote: »

Xantrex 6048 Inverter
Xantrex 80A MPPT 600V Charge controller
4600 watts of Conergy Panels (20 panels at 230w each)
24-85-25/ 48 volt / 1608ah @ 20hr rate / 4000 lb Deka D-Series forklift battery

Hi, this looks like quite a nice system, and I must disagree with most of the comments so far about the "too much battery" problem. In my opinion, the phrase "too much battery" should be replaced with "too much load", the load should be matched to the solar production. You then have some margin to choose the battery bank size based on how much you want to spend, how many days of autonomy you need and how long you want them to last.

Since you already have the battery, you gain nothing by making it smaller!

fowlerrudi wrote: »

The questions:
Will I need a generator to EQ this battery?

That depends on how much you're drawn it down the night before. If it's only been discharged slightly then you could EQ from solar alone.

fowlerrudi wrote: »

How much generator do I need to EQ this bad boy?

See above

worst case scenario, battery drawn down 50% in winter and you'd like to EQ on the next sunny day, so 1608Ah / 2 = 1304Ah * 1.2 charge factor = roughly 1564Ah to replace. If you could get the batts up to EQ voltage by midday you should have enough solar to maintain EQ voltage, assuming you're not drawing too much from the loads. So if you could get to 90% of the 1564Ah with the generator you'd be well on your way. The target then is to get 1400Ah into the battery before midday and without overheating the battery. With the 100A charger limit on the XW, that means a gen runtime of 14 hours! And a generator that can output 100A * 48V = 4.8kVA + peak loads, so anything from 6kW - 10kW could work, depending on your loads.

You could get away with a smaller gen, but then you can't discharge the battery so deeply - or you'd have to leave the gen running for many more hours to get the battery charged after a deep discharge.

fowlerrudi wrote: »

Battery was at 55.5V and showing fully charged, after a night of working at the house, it was down to 49V.

You can't tell the state of charge from the voltage. Measuring the SG will give you a much better idea of the state of charge.

fowlerrudi wrote: »

I am however concerned that I have too much battery.

Since you already have it, this is not a problem, just don't draw it down as deeply. It'll last longer too with shallower discharges!

fowlerrudi · October 2012

Re: Money and No Brains

If I take away 10% of my battery at night, then put it back the next day via solar, and do this everyday, then this will be fine correct?

Lets say I have 4 days of cloud, I take the battery down to 50%, it will take me 2 sunny days to get this battery up to 100% again without running a genny. Is this okay?

I will not be able to charge at 10%.

The only disadvantages to a bigger battery at the time of me buying it was - 4000 lbs, need tractor to move, and the self discharge of a forklift battery is higher than an expensive Surrette.

I should be able to EQ with solar no problem, or with a 3000 watt generator - because from what I understand I only need higher voltage and not any amperage for this event.

I am full of book knowledge, I have no experience, other than living off a little 2000 watt generator for the past 4 years (which is humming in the background as I type this). Everything I have ever read says that the smaller the depth of discharge on the battery, the more charge cycles you will get. I am reviewing this now in Bill Kempt's book 'Renewable Energy Handbook." However I am being told now that discharging is good for it? I read on this forum elsewhere that a good discharge once a month would be good for the battery. I also read on the paperwork that came with my refurbished battery that it was good to discharge it a lot. So in my head I am thinking that a large battery only coming down 10% a day will last longer than a battery that is brought down 50-80% daily.

What's done is done, I figure if I have to scrap this battery in 5 years, after selling the battery(lead) for scrap, it will have cost me about $1700 bucks. $1700 bucks doesn't buy many surrettes.

I feel like the verdict of our conversations is that I have a decent system, which should be able to provide me with an easy 10 KWhr per day, however what I am being told is that I have a monster battery, which won't last because I will be draining it down 80% each day and will never have to gunpowder to bring it back up. This is true if my loads were high, but they aren't. (or aren't supposed to be - I haven't moved in yet remember).

So am I going to fry this battery #1 - by not charging it at 10% rate? #2 Will I fry it because I don't discharge it enough often? Should I do #1 and #2 once a month or once a week using a monster generator?

If I did take my battery down 80% so there is 20% remaining, it would take me 6 days @ 10KWhr a day (since 80% @ 20hr discharge means I have around 62KWhr of useable energy). But there are 144 hrs in 6 days, so the 20 hr discharge rate shouldn't be used. I know on the 5000 series surrette batteries the difference between the 20hr and 100hr is around 40% additional amperage - this means that instead of 1608AH @ 20hr I should have around 2251AH @ 100hr. Ignoring any losses such as charger inefficiencies and self discharge of course.

Am I thinking too much?

Cariboocoot · October 2012

Re: Money and No Brains

Problem: electrolyte stratification.
Particularly found in large batteries that don't get used much and so never get a good, long Absorb stage to remix the electrolyte. Accelerates the rate of sulphation leading to premature battery death.

Problem: occasional deep discharging down to 50% requiring long time (days) to recharge due to insufficient charge capacity.
This can result in the battery spending too much time below 70% SOC which accelerates the rate of sulphation leading to premature battery death.

Yes you can keep a battery burbling along at 10% maximum DOD for a long time. But see above.

Anyone and everyone is free to disagree with me.

fowlerrudi · October 2012

Re: Money and No Brains

I did a little reading - found this: http://www.necon.co.za/battcare.php?w=1366&h=768

Perhaps the "boost charging" feature on the XW Charge controller could be of use?

"Boost charging allows for better utilization of flooded lead acid batteries under
moderate cycling in off grid applications. Boost charging encourages a short
duration charging voltage—above the gassing voltage—at the beginning of the
absorption charge state. Testing has shown that boost charging improves battery
performance by providing a regular mixing of the liquid electrolyte. Boost
charging specifically discourages capacity-robbing acid stratification and plate
sulfation.
Boost mode charging can be enabled by selecting the custom battery type and
then setting the bulk voltage higher than the absorption voltage. The multi-stage
charge algorithm then attempts to use the higher bulk voltage for the first hour of
the absorption stage – unless interrupted by the max absorption timer or exit
current threshold."

Photowhit · October 2012

Re: Money and No Brains

Hi Welcome to the forum (as a contributor)

I looked into reconditioned fork lift batteries before I purchased my new forklift battery. We have a fellow here who has a couple of them. While looking into them i discovered they have a higher self discharge rate and typically have 80-90% of the new capacity, so you may not have the full capacity you think you have. Your concern with Equalizing is well founded, Forklift batteries need to be equalized particularly in a shallow discharge application, to help mix the electrolyte. I personally think it's an either or thing with the deep discharging, the idea being it helps mix the electrolyte.

If you can get the battery topped off the equalizing will not be a problem even from your current array. It appears you have no battery monitor, I though it was integrated in the Xantrex system? This would be nice to help keep track of where your at on the battery. I think you have the right idea, use this as your 'starter battery' much as we recommend cheap golf cart batteries for smaller systems, you'll learn alot and if you trash them your not out a ton of money.

New Brunswick, Canada makes me think of long dark winters, I would thing a quality alternative charger will be needed, I would shoot for not allowing them to discharge past 50% (rated on 80% of a new batteries capacity) and fully charging them at least once a week. If you charge in the morning you likely can equalize every month on sunny afternoons after the batteries have reached full charge a(so long as you don't have big loads during that time.)

Checking battery state of charge by voltage is difficult, if you have some charging going on it's likely to read high, if it's dark out and you put a load on it it'll drop like a rock. Next time you see it well out fromwhere you think it should be let the system rest for an hour and likely you'll see the voltage move toward where you think it should be... but a battery monitor is a good idea...

stephendv · October 2012

Re: Money and No Brains

fowlerrudi wrote: »

If I take away 10% of my battery at night, then put it back the next day via solar, and do this everyday, then this will be fine correct?

Yep, it's what I do and it's what the DoD vs. cycle curves tell you will give the battery a good long life.

fowlerrudi wrote: »

Lets say I have 4 days of cloud, I take the battery down to 50%, it will take me 2 sunny days to get this battery up to 100% again without running a genny. Is this okay?

I don't think you'll be able to get it charged back up on solar alone in 2 days because you'll be running your normal loads during those days as well- so depending on those loads it could take 3 days to get charged back up. How much this will effect battery life- I don't know. I haven't found any studies showing how fast sulfation builds up, so it's difficult to tell how much effect a 1 day vs a 3 day recharge will have on long term battery life. You could do a full absorb and a short EQ after a deep discharge to counter act sulfation effects.

fowlerrudi wrote: »

I will not be able to charge at 10%.

In my opinion this is not a problem as long as your 50% DoD discharges are infrequent events.

fowlerrudi wrote: »

I should be able to EQ with solar no problem, or with a 3000 watt generator - because from what I understand I only need higher voltage and not any amperage for this event.

You will need a good amount of current to get it to the EQ voltage if you're going straight from bulk to EQ (which is the default for most charge controllers), once at the EQ voltage the current will taper off. Going from a battery with a DoD of 10% to EQ voltage will not take a lot of current, you might spend an hour in bulk and then another hour or two to reach the EQ voltage, then you need to stay at that voltage for 2-6 hours depending on whether your cells are all equal or not. So doing this in winter without a gen will be challenging.

fowlerrudi wrote: »

However I am being told now that discharging is good for it? I read on this forum elsewhere that a good discharge once a month would be good for the battery. I also read on the paperwork that came with my refurbished battery that it was good to discharge it a lot. So in my head I am thinking that a large battery only coming down 10% a day will last longer than a battery that is brought down 50-80% daily.

I don't understand why discharging a battery would be better for it in the long term. None of the battery manuals I've read recommend discharging the battery deeply. What I have read is that if you leave an FLA battery on a prolonged float charge only, then it gets damaged because of stratification. But you can counter act this by doing a monthly EQ charge, without first discharging the battery.

Do you have any references to material or studies that suggest periodic discharging actually prolongs battery life?

fowlerrudi wrote: »

So am I going to fry this battery #1 - by not charging it at 10% rate?

Just to be clear this 10% rate is not important because of the charge rate itself, it's just a shorthand for saying "can you recharge your battery quickly after you've discharged it".

fowlerrudi wrote: »

#2 Will I fry it because I don't discharge it enough often? Should I do #1 and #2 once a month or once a week using a monster generator?

IMHO no. Avoid deep discharges. Do a long normal EQ once a month. Do a short EQ after deep discharges, e.g. do a normal absorb charge and once it's done do an EQ for 30 minutes, check SGs to see that the battery is fully charged.

fowlerrudi wrote: »

If I did take my battery down 80% so there is 20% remaining, it would take me 6 days @ 10KWhr a day (since 80% @ 20hr discharge means I have around 62KWhr of useable energy).

That's 10kWh/day into the battery, excluding your loads during those days. And if you're thinking of using solar only, that's also excluding the recharge for the nightly discharge during each of those 6 days.

fowlerrudi wrote: »

But there are 144 hrs in 6 days, so the 20 hr discharge rate shouldn't be used. I know on the 5000 series surrette batteries the difference between the 20hr and 100hr is around 40% additional amperage - this means that instead of 1608AH @ 20hr I should have around 2251AH @ 100hr. Ignoring any losses such as charger inefficiencies and self discharge of course.

Think 40% between C20 and C100 is too much. Attached an approximation of converting between the different C values. A battery monitor would calculate the recharge Ah based on what you took out of the battery. I.e. use the peukert effect to determine how much you took out, then multiply by 1.2 to give you a rough idea of what you need to put back. The time it takes you to put it back isn't subject to peukert as far as I know.

Attachment not found.

fowlerrudi wrote: »

Am I thinking too much?

The opposite usually causes more problems

stephendv · October 2012

Re: Money and No Brains

fowlerrudi wrote: »

"Boost charging allows for better utilization of flooded lead acid batteries under
moderate cycling in off grid applications. Boost charging encourages a short
duration charging voltage—above the gassing voltage—at the beginning of the
absorption charge state. "

Yes! If you check the charging profile used by modern forklift chargers, they use an IUIa profile: Bulk, shorter absorb, charge to EQ voltage and stop when EQ voltage is reached. This is done daily, because forklifts do a deep discharge every day 5-7 days a week, and the batteries need to be recharged overnight ready for the next shift: the IUIa curve is the fastest way to recharge the battery.
That short EQ helps mix the electrolyte and reverse sulfation as I mentioned in the previous message.

Don't know about the boost feature of the XW, none of the other popular charge controllers have this feature. I kind of did the same thing with my morningstar by configuring it to do a 5 minute EQ every week. This meant it went straight from bulk to EQ voltage for 5 minutes, then dropped into absorb for another 3 hours. The purpose of the short EQ is to mix electrolyte and to make sure the battery reaches 100% SoC in case the daily charges were too short.

Volvo Farmer · October 2012

Re: Money and No Brains

stephendv wrote:

I don't understand why discharging a battery would be better for it in the long term. None of the battery manuals I've read recommend discharging the battery deeply. What I have read is that if you leave an FLA battery on a prolonged float charge only, then it gets damaged because of stratification. But you can counter act this by doing a monthly EQ charge, without first discharging the battery.

Do you have any references to material or studies that suggest periodic discharging actually prolongs battery life?

I grabbed this from Rolls before they took all their material off the website. It is the only mention I have ever seen of it though. http://www.frontier.net/~bobwenn/anotherpower/Rolls%20Surrette%20charging.pdf

I will leave the link active for a couple days before I kill it.

"Once every three months the bank should be discharged to
the low voltage set point before starting the generator. This is usually dependent
on the cut-off of the inverter which is usually 11 volts on a 12V system. The
batteries are designed to be cycled and a deeper discharge forces electrolyte
deeper into the active plate material and helps open up fresh reaction sites."

waynefromnscanada · October 2012

Re: Money and No Brains

Thanks for that Volvo Farmer. Very interesting reading.

stephendv · October 2012

Re: Money and No Brains

Volvo Farmer wrote: »

"Once every three months the bank should be discharged to
the low voltage set point before starting the generator. This is usually dependent
on the cut-off of the inverter which is usually 11 volts on a 12V system. The
batteries are designed to be cycled and a deeper discharge forces electrolyte
deeper into the active plate material and helps open up fresh reaction sites."

Good stuff, thanks for sharing!

techntrek · October 2012

Re: Money and No Brains

A note on the "50% rule". That rule exists because over time that is the best cost-to-benefit ratio for most lead acid batteries. It is the best balance between wear and cost. Only discharging to 10% most of the time will definitely extend the life of lead acid batteries but it increases your up-front costs since (as others have already said) you end up with way more battery than you need for your loads. But since batteries only have so much life in them no matter how much or how little they are used, you end up getting fewer total amps per dollar out of them. Going the other direction, discharging to let's say 70% most of the time, gives you a smaller up-front cost since you have purchased a smaller battery, but the deeper discharge will shorten their life so you must replace them sooner. You still end up getting fewer total amps per dollar over time. 50% DOD is the best balance to give you the most amps per dollar over time. If you have studied Economics it is like finding the best profit point on the curve.

So from a system standpoint you will be OK only discharging to 10%, the battery will last you much longer, as long as you replace that 10% on most days.

BB. · October 2012

Re: Money and No Brains

Since this document (text version) has been posted publicly for ~5+ years now (here), I will upload this one (from Volvo Farmer) here to reference the original.

If there are any issues, please feel free to contact me.

Regarding the 5-13% recommended rate of charge... When you are at ~6% rate of charge, that does not leave you much room to use power during the day (fridge, computer, electronics, vacuuming, etc.).

The closer you are to 5% rate of charge, the more careful you have to be to not use much power while the sun is up and the battery is charging (once charged, certainly feel free to use power when the battery is in float). If you get to float at least a few times a week, you are probably doing fine.

-Bill

niel · October 2012

Re: Money and No Brains

actually 50% is somewhat of an arbitrary number as a good overall number to insure some life with some heavy discharging that isn't likely to reverse cells on new as well as old batteries. if i recall rightly the best point for life versus amps produced is in the area of 65% to 70%, but this can also vary for different batteries and different ages so it's still arbitrary.

Money and No Brains

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