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Forum Index : Solar : Dual battery setup for off grid solar
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| Godoh Guru  Joined: 26/09/2020 Location: AustraliaPosts: 667 |
We have a 24 volt off grid solar system. At the moment if the cloud cover is heavy and no solar energy is generated we have 3 days max of power in the batteries before they drop to 70% charge. Our batteries are BAE valve regulated lead acid batteries at 24 volts and 660 amp hour. We are thinking of getting another bank and using a switching system to change over banks when we have cloudy weather. ( in winter we are in the clouds a bit). It is rare that we have no power input at all but it does happen, recently I had to resort to a small petrol motor and car alternator to top the batteries up. The spec sheets for the batteries say that if we only use the top 30% of the capacity they should last around 15 years. That would be nice. So I have ordered two 500 amp kilovacs to do the switching, but I need both banks to be kept charged when the solar input is enough. I was thinking of using a voltage controlled relay to switch the kilovacs, so that when one bank is fully charged then the other would be switched on to supply power and also be charged up. I am just wondering if anyone here has ideas on the best way to run the system. Our original bank of batteries are now about 8 years old and still in good condition, obviously I don't just want to parallel a new bank to them. Thanks Pete |
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Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1277 |
I have read in a few place's that some run a LFP battery along side the Old Lead acid to extend the life of the Lead Acid by quite a margin, the LFP battery will do most of the work, when they get down the Lead acid takes over and carries on making them an ideal when the weather isn't great. Actually I have this setup in the shed but only for welding and other high surge stuff so have never run it flat to see the FLA batteries take over. Probably something to measure the SOC on either pack and programmed to switch the relay. Cheers Aaron Off The Grid |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1216 |
One of our systems I built as an interim setup until the big system goes online, was a 24v sealed Lead Carbon 300 AH bank, it had proven to be a tad small due to users piling on more and more loads, then leaving things running over night when not in use. As the bulk charge voltage was almost 28v I decided to augment its capacity by paralleling it with a 24v 200AH bank of Lifepo4 cells re-purposed from dismantled 48v 100AH battery packs I had rescued. This has proven to be hugely successful, the Lifepo4 cells only get to 3.5V each, which is ideal, they do all the work, supplying power to the inverter until their voltage drops where the L.Carbon sealed cells take over. I use one of those Kilovacs to isolate the Lifepo4 bank from the L.Carbon bank when their voltages start dropping near 22 volts, the BMS senses individual cell voltages and switches the bank out via the contactor, it re-enables it again when the Lead cell voltage > 24 volts. So that is maybe another option, a new bank of 24v Lifepo4 cells will do all the heavy current grunt work, only drawing down on your older Lead as they start getting discharged. Here in NZ, Lifepo4 banks are less expensive than any form of lead. Cheers Mike Edited 2026-06-20 20:13 by Solar Mike |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 667 |
Thanks Aaron and Mike, that is an option I hadn't considered. Apart from the LFP battery in the car I have no experience with lithium batteries. Lifepo4 batteries seem pretty expensive here, but I will have a look and see what is available. Any suggestions on which brands are worth looking at? Pete |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1216 |
I would purchase individual 314 or 650AH cells and make your own 24v bank, 8 cells in series as the least expensive option; look on Off Grid Garage , its Australia based, he has tested all types of Lifepo4 batteries and has links to source suppliers. Mike |
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Bryan1 Guru Joined: 22/02/2006 Location: AustraliaPosts: 2100 |
Pete looking at this quote I got for a 24 volt 600AH new forklift battery $3533+gst is a good price for a battery that will last you a decade or more. Now on my house array got 1.9Kw of 190 watt panels going into a victron 100/50 MTTP that tops out at the max 50 amps which is a good charging scheme and on a sunny morning the batteries are in float mode.  On my shed array I did find as that 4KVA genset only outputs 220 volts AC it limits the current on my forklift battery charger to around 30 amps. So I put 3 off 330 watt panels leaning on the east side of my shed and in full sun I'm getting that 30 amps which intern has kept my shed battery topped up. It pay to go ask the electric forklift companies and keep hounding them for a battery that has come off lease as when the electric forklifts go on a new lease they have to have a new battery. You may just find one and get lucky mate Regards Bryan |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 667 |
Thanks Mike looks like I have some watching and reading to do. I do have a 500farad capacitor bank in parallel with my batteries, they definitely help with big starting currents. I will have a look at the Off Grid Garage site later and see what he has to say. Thanks too Bryan, I will check them out, the price sounds really good. Pete |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 667 |
Hi Mike I have another query on your Lead Carbon and LifePo4 setup. It seems that you have both banks connected in parallel until the voltage falls to 22 volts, then the Lead Carbon is the only battery running the system. I have read a bit about charging LIFE P04 batteries and I know that my MPPT regulators have different settings for charging different battery types. So will the charger voltages that my regulators are set for ( 28.8 volts) damage or undercharge the Lithium batteries. thanks Pete |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1216 |
Lifepo4 cells top bulk voltage is 3.6v, generally however its good practice to only go to 3.5v/cell or 28 volts, this voltage suits L.Carbon as at 20°C that is also its max bulk voltage. If your charger is accurate and never exceeds 3.6/cell, they will work ok, I limit our CV period following bulk to 2 hours in the combined bank; if it was a Lifepo4 only bank, I limit the CV period to however long it takes for the BMS to achieve cell balance, usually 20 minutes or less. Mike |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 667 |
Thanks Mike, We will do more studying and see which way we go. I still need to make a base for the batteries to sit on and do some work on sealing up the carport area where they will go. I have been looking at the Off Grid Garage videos, he seems to be doing reports on many different banks. Cheers Pete |
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| Bryan1 Guru Joined: 22/02/2006 Location: AustraliaPosts: 2100 |
Pete as the forklift batteries are so heavy all I did was use some 1" diameter black bar to move the battery around, I used 8 of them and as one became free it went under the front. Now when I need to change direction I just angle the forward bar and it is easy to slide the battery sideways on the rods. When the battery is in position the rods under stay there for the next time it needs to move. The main thing I like about these batteries one can know the state of charge by just checking the SG and when charging via a genset the SG rises once the batteries hit 29 volts then after around an hour of the soakage charge does finish off the charge. The batteries also have a auto watering system which makes it easy to keep the levels right and basically once it's setup it's there for long time of service. Regards Bryan |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 667 |
Hi Bryan, I did have a look at forklift batteries, but here there is no way to move something like that about. I have moved slow combustion stoves about on pipe rollers before but here I don't have a slab to put the batteries on. Just a sloping carport with gravel on the ground. So moving a very heavy bank of batteries up the slope in the carport won't work. i am leaning towards just duplicating what i have now, for simplicity. I like being able to check the charge easily but the gassing off won't work either. My batteries are near where the inverters are mounted, I have seen what sulphuric acid fumes do to circuit boards and that is not pretty. Pete |
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