Two battery banks feeding one load?

Iceni John · August 2014

I'm getting close to planning the wiring for my bus conversion's PV system, specifically from the house batteries to their loads. Because I have about 2kW of panels on the roof, they will feed two Morningstar TS-MPPT-60 charge controllers that will each charge four golfcart or L16 batteries, so up to that point I will have two separate systems providing house power. (The bus's starting batteries and alternator will be completely separate from the house system, and will remain unchanged from now.) My question is how best to combine these two separate battery banks to one common load (the main DC panel and the inverters), without any risk of one bank back-feeding into the other. Vanner makes battery isolators, but I don't want to use something that wastes so much power - even Schottky diodes are very inefficient, especially at the high current levels I may be drawing from the batteries to power my inverters.

Can I have in effect two independent battery banks feeding one single load at all, and if so what is the best approach for me to do so? Is there some magic doohickey out there that will do what I want?

Just in case anyone is wondering, I have two rows of four panels on the roof, each row on either side of a central walkway, and each panel can be independently tilted. However, if one side of panels is laying down against the roof at about 22 degrees down (my bus has a very curved roof), in the winter the other side of panels can be raised up to 45 degrees. This obviously means that each row of panels will then be producing slightly different amounts of power due to their different tilt angles, hence the need for two charge controllers. My reasoning is that if I have two CCs, why not also have two separate battery banks to have redundancy, and to allow me to replace just four batteries at a time, or even have different batteries in each bank?

Has anyone else here done anything similar? I've searched through this forum, but not found anything pertinent so far.

Thanks, John

zoneblue · August 2014

Re: Two battery banks feeding one load?

Im thinking that makes things more tricky. You cant connect two banks on one load without the banks becoming effectively parallel, and that will only work if the banks are identical. At that point you have what most people have and thats multi strings /controllers on the one bank. The alternative is some kind of switch arrangement that will possibly drive you nuts.

I would also add that 2kW of pv should be on one controller only, unless you have real serious shading problems. Idle losses, etc etc.

Iceni John · August 2014

Re: Two battery banks feeding one load?

zoneblue wrote: »

Im thinking that makes things more tricky. You cant connect two banks on one load without the banks becoming effectively parallel, and that will only work if the banks are identical. At that point you have what most people have and thats multi strings /controllers on the one bank. The alternative is some kind of switch arrangement that will possibly drive you nuts.

I would also add that 2kW of pv should be on one controller only, unless you have real serious shading problems. Idle losses, etc etc.

Yes, that's exactly my dilemma - the two battery banks may be different, and I plan on buying their batteries at different times to stagger their inevitable replacement purchases.

Remember that this setup is on an RV / motor home, so therefore it's 12V. A mobile installation like this is about the only time one can justify not going to 24V or 48V. I do NOT want fan-cooled charge controllers, so that effectively leaves only the Morningstar and Xantrex - for my needs the TS-MPPT-60 will be better, but it cannot handle 2kW of panels into 12V batteries. Plus, my two rows of panels may produce slightly different power at some times of the year depending on their different tilt angles, and that's a no-no into one charge controller. Being on the bus roof, there will be absolutely no shading from anything on the bus!

Thanks, John

stephendv · August 2014

Re: Two battery banks feeding one load?

One of these products might be of interest: http://www.smartgauge.co.uk/products.html

vtmaps · August 2014

Re: Two battery banks feeding one load?

I wonder if it's possible the combine the power of the two banks at the AC level... that would probably entail using one battery and inverter to set up a minigrid, and using a hybrid inverter on the other battery.

--vtMaps

Iceni John · August 2014

Re: Two battery banks feeding one load?

I think I may have found something that could, maybe, perhaps, just work: http://www.allbatterysalesandservice.com/browse.cfm/4,3002.html The blurb isn't very clear exactly what it does, but the circuit diagram helps explain it better (scroll down to figure 3 on page 2): http://www.allbatterysalesandservice.com/graphics/INSTRUCTION_SUREPOWER_Marine_Isolators.pdf

Yes, I know it will absorb at least half a volt because it's diode-based, but it would be between the batteries and loads so their charging won't be affected. Even the model 3203's 120A rating may be slightly too low for me - I'm assuming that it can put a maximum of 120A of combined inputs into its center output (shown on the diagram as an auxiliary battery, but for me would instead be the main DC distribution panel).

The only other option I can think of for now is having two of these 2-output Schottky (I assume) isolators from Sterling Power, one for each battery bank, and combining all four of their outputs to my DC load center: http://sterling-power-usa.com/lowvoltagedropisolators.aspx At least this way each battery bank has its own isolator for redundancy, and these Sterling isolators should drop less voltage than the Sure Power isolator, plus I can get a choice of amp ratings to better suit my needs.

Any ideas on this, anyone? I feel like I'm bravely (OK, maybe blindly) going where I don't think anyone's gone before - I can't believe nobody else has two separate battery banks feeding one shared load? If it wasn't a mobile setup for which 12V is the only realistic way to go, I would have a 24V or 48V system instead, but I'm stuck with having 12V.

Stephen, the SmartGauges interest me a lot. They make a lot more sense than the usual Trimetric or similar gauges. Too bad that two of them would be so expensive here. However, do I really need that level of battery monitoring if I have so much PV power to keep my batteries fully charged every day? Maybe simpler gauges like this would be sufficient for me: http://www.solar-electric.com/mnbcm.html Anyway, first things first, they're way down my list of priorities for now!

Thanks again, John

waynefromnscanada · August 2014

Re: Two battery banks feeding one load?

Iceni John wrote: »

I plan on buying their batteries at different times to stagger their inevitable replacement purchases.

While this would greatly ease the burden of purchasing the whole battery bank at once, it will cost more in the end. Reason being, for the good of the batteries and to not shorten their lives, they should all be of the same age. Generally putting new batteries in a bank of older ones will result in the new batteries prematurely deteriorating to the level of the weakest/oldest batteries in the group. Therefore it is accepted practice to have all batteries in a bank very close to each other in age, preferably even with the same date code.

vtmaps · August 2014

Re: Two battery banks feeding one load?

Iceni John wrote: »

do I really need that level of battery monitoring if I have so much PV power to keep my batteries fully charged every day?

Do you need a gas gauge on your car if you stop at the gas station every day?

Whatever battery monitor you buy, remember that they can give a treacherous, false sense of security. Be sure to use an hydrometer to calibrate (and verify) your monitor. And check SG every time you add water, to see if the monitor still agrees with SG.

--vtMaps

Iceni John · August 2014

Re: Two battery banks feeding one load?

waynefromnscanada wrote: »

While this would greatly ease the burden of purchasing the whole battery bank at once, it will cost more in the end. Reason being, for the good of the batteries and to not shorten their lives, they should all be of the same age. Generally putting new batteries in a bank of older ones will result in the new batteries prematurely deteriorating to the level of the weakest/oldest batteries in the group. Therefore it is accepted practice to have all batteries in a bank very close to each other in age, preferably even with the same date code.

Sorry, what I meant is that each bank of four batteries would be bought at one time, obviously, but I could stagger the replacement of each entire bank, i.e. buy four batteries, then a few years later buy the second bank while the first bank is about halfway through its life. This way I'm buying four batteries more frequently, instead of eight batteries less frequently. My idea is that a sudden catastrophic failure of one battery won't leave me without power, because I'll still have a hopefully-newer second bank in reliable use. And because each bank of batteries is a different age to the other, and because each bank is separately charged by its own charge controller (because the two rows of PV panels may well be giving different power depending on their tilt), there's nothing stopping me from even using different types of batteries in each bank. I could conceivably have four golfcart batteries in one bank, and four L16s in the other. This is why it's vitally important that I isolate each bank from the other where they feed into the main DC system.

Thanks, John

Alaska Man · August 2014

Re: Two battery banks feeding one load?

We used to switch between banks in a fishing boat I worked on. Do you have to have both banks feeding at once? A battery switch is pretty cheap.

stephendv · August 2014

Re: Two battery banks feeding one load?

Iceni John wrote: »

Stephen, the SmartGauges interest me a lot. They make a lot more sense than the usual Trimetric or similar gauges.

The reason I listed the smartguage link wasn't for the battery monitors it was for the split charger that they produce called the Smartbank: http://www.smartgauge.co.uk/smartbank.html. Their split charger was designed for a very similar use case to yours: 2-4 batteries and a single charging source.

Iceni John · August 2014

Re: Two battery banks feeding one load?

Alaska Man wrote: »

We used to switch between banks in a fishing boat I worked on. Do you have to have both banks feeding at once? A battery switch is pretty cheap.

That's for using either one bank or the other at one time. I want to draw from both banks all the time, but without one bank able to back-feed to the other.

I guess some sort of Schottky-based devices are in my future!

Thanks, John

BB. · August 2014

Re: Two battery banks feeding one load?

Diodes add voltage drop (0.2 to 1.0 volts or even greater). When you are are taking about significant current (100 amps on 12 volts is 1,200 watt microwave), you are looking at 20-100 Watts (P=I*V=0.2v*100a=20w)--That is a lot of wasted energy, for a 12 volt battery bank that is about your entire "margin" for wiring voltage drops, and a lot of heat sinking on the diodes (and the heat sink will usually be at +12 VDC).

Plus diodes do fail--Sometimes they fail shorted or sometimes open... So you have to keep a watch that they are performing as needed.

There are standard systems that use separate battery banks. Generally this would be a pair (or more) of off grid AC inverters that can parallel (or series) connect their AC output. The inverters will draw from their own battery banks and supply common power (shared per configuration/energy usage) on the AC side.

Maintaining separate battery banks is generally something I would suggest to avoid. It means that you have separate solar panels/AC charging--Which means you cannot sure energy across banks easily (i.e., if one bank is low, you cannot easily take power from the other bank/solar array).

And your chances of failure can increase. More components, and if you need two banks (such as to AC inverters in series for 120/240 VAC--inverters designed to do this function) means that a failure in one bank will cost you the 240 VAC function or 1/2 the output of your inverter(s).

Having mixed age of battery banks--Say your battery bank lasts 7 years. Two bank 3.5 years apart means that you are now doing major diagnostics/servicing on your battery bank every ~3.5 years vs the whole bank every ~7 years. Sort of forces you to "tinker" with your battery bank/power system 2x as often. (batteries do fail early, either single cells/batteries or more--Depending on what/why is failing--Mfg. or owner mistakes, etc.).

-Bill

Two battery banks feeding one load?

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