Downsides to oversized battery bank?

Hello!

I'm adding a 12V system to my van, and the best choice appears to be the 280Ah lifepo4 DIY batteries.

Realistically, I only need about 20 to 30Ah at the moment - a fan, some lights, a diesel heater. Down the road, maybe a year or two, I'll be adding higher-draw items such as a fridge or cooler, coffee pot, etc. There will also be many days on which I don't use any power at all.

Is there a downside to having so much excess capacity? If I'm discharging only a tiny bit, and then recharging, is this bad for LFPs? Anything else I should consider?

Many thanks!

I can't think of any downside to oversizing LFP. Shallower depth of discharge means they'll last longer.

Ai4px said:

I can't think of any downside to oversizing LFP. Shallower depth of discharge means they'll last longer.

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Thank you!

The main concern, and you're discussing exactly what I have in my van, is the fact that the lithium batteries will take all the current your alternator can deliver when they're low. A big battery can easily overload an alternator and overheat it, whereas a lead/acid battery just won't take as much current. A 280 amp batttery could easily draw 100-150 amps when it's low if nothing slows it down.

The usual answer is to not charge the battery directly from the alternator, but to feed a dc/dc converter that can be programmed to a maxiumum charge rate low enough to protect your alternator. I use a solar charge controller with a built in dc/dc charger, as my alternator is only 60 amps. The charger is set to a maximum charge rate of 30 amps, allowing enough for the van to operate while also charging the lithium.

Once that's taken care, lithium is a lot less hassle than lead/acid, you never have to worry about it being fully charged every day, they're happy (happiest in fact) to sit at half charge for months, which would kill a lead/acid battery quickly.

__mike__ said:

Is there a downside to having so much excess capacity?

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Cost and weight. Why lug around far more than you need? Buy a bigger battery if and when you actually need it.

I would keep it floated at 50% for longer life and temporarily charge to 100% every 3-6 months.

This is a petty complaint; not even really a reason.

Takes too long for a BMS to read SOC correctly. I have a JBD/Overkilol BMS and one of the two had only cycled twice and continues to read SOC incorrectly. I have had it about two months, but only. Used it three weeks. I think if the battery were smaller it would have been through enough charge cycles to read correctly. At some point it will read SOC correctly.

Bvillebob said:

The main concern, and you're discussing exactly what I have in my van, is the fact that the lithium batteries will take all the current your alternator can deliver when they're low. A big battery can easily overload an alternator and overheat it, whereas a lead/acid battery just won't take as much current. A 280 amp batttery could easily draw 100-150 amps when it's low if nothing slows it down.

The usual answer is to not charge the battery directly from the alternator, but to feed a dc/dc converter that can be programmed to a maxiumum charge rate low enough to protect your alternator. I use a solar charge controller with a built in dc/dc charger, as my alternator is only 60 amps. The charger is set to a maximum charge rate of 30 amps, allowing enough for the van to operate while also charging the lithium.

Once that's taken care, lithium is a lot less hassle than lead/acid, you never have to worry about it being fully charged every day, they're happy (happiest in fact) to sit at half charge for months, which would kill a lead/acid battery quickly.

Click to expand...

I have a 28V 50A aircraft alternator on my van. I use a potentiometer to manually control field current on my Transpo VR2400 external voltage regulator. I monitor voltage, current and alternator rear housing temperature.

RV10flyer said:

I have a 28V 50A aircraft alternator on my van. I use a potentiometer to manually control field current on my Transpo VR2400 external voltage regulator. I monitor voltage, current and alternator rear housing temperature.

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I take it this is a second alternator?

My truck has space for alternator #2 that is not installed currently. Have not yet used enough power to think about using it. Always a thought if my power needs go up.

Many thanks to you all! This is very helpful.

Bvillebob said:

The main concern, and you're discussing exactly what I have in my van, is the fact that the lithium batteries will take all the current your alternator can deliver when they're low. A big battery can easily overload an alternator and overheat it, whereas a lead/acid battery just won't take as much current. A 280 amp batttery could easily draw 100-150 amps when it's low if nothing slows it down.

The usual answer is to not charge the battery directly from the alternator, but to feed a dc/dc converter that can be programmed to a maxiumum charge rate low enough to protect your alternator. I use a solar charge controller with a built in dc/dc charger, as my alternator is only 60 amps. The charger is set to a maximum charge rate of 30 amps, allowing enough for the van to operate while also charging the lithium.

Once that's taken care, lithium is a lot less hassle than lead/acid, you never have to worry about it being fully charged every day, they're happy (happiest in fact) to sit at half charge for months, which would kill a lead/acid battery quickly.

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I'll definitely include a b2b charger in there, thank you! My alternator is rated for 230A, but I think a 40A DC-DC charger will be big enough for me, and not too expensive.

rmaddy said:

Cost and weight. Why lug around far more than you need? Buy a bigger battery if and when you actually need it.

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A fair point!

RV10flyer said:

I would keep it floated at 50% for longer life and temporarily charge to 100% every 3-6 months.

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Sounds good, thanks!

chrisski said:

This is a petty complaint; not even really a reason.

Takes too long for a BMS to read SOC correctly. I have a JBD/Overkilol BMS and one of the two had only cycled twice and continues to read SOC incorrectly. I have had it about two months, but only. Used it three weeks. I think if the battery were smaller it would have been through enough charge cycles to read correctly. At some point it will read SOC correctly.

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I would never have thought about this! I takes a few cycles to calibrate, then? That's good to know, so I don't worry when it's initially wrong.

Again, thanks for taking the time to help me out.

Even with limited needs I would still go with the 280 minimum. If you short charge the battery (13.60 volts?) the short cycles will not matter.

If I could ask another question: what is the recommended charger these days that would let me choose to short charge when I want to, and full charge at other times? I earlier said 40A but I could safely go up to about 75A at 12V.

Thanks!

Full charge or “short charge”. I think you mean you only want to fill the battery to, say 60% SoC on a “short charge” and 100% on a “full charge”? Have I understood the question correctly?

LFP has such a flat voltage curve that it will be near impossible to use the voltage of the pack to trigger something at 60 or 70 or 80%… you’ll need a BMS that tracks amp hours in an out of that level of granularity.

I have a friend with a SolArk and we used a relay that triggers on SoC to inhibit his generator from running. His issue is that once SolArk decides to start the generator, it won’t stop the generator until the pack is 100%… so we use a programmable aux contactor to disable the generator run signal if the SoC is above 60%. THere’s no need to spend fuel to charge the battery to 100% when the sun will be coming up in a few hours.

So does your BMS have any relay outputs for SoC? Or do you even want to be able to charge to 60% or 70%?

I bought this 75 amp 12 volt 37.5 amp 24 volt charger off Will’s recommendation:


You can get it cheaper by going to the DIY blueprints and he has a link where you can get it for less $$

I installed it in my RV and leave it off unless a cloudy day comes along and turn it on. So far, I’ve only ran it for 40 minutes to return 22 amps at 24 VDC. This is the only one I found for a 15 amp 120 VAC jack that puts out that much charging amperage,

I’m not sure what you mean by short charge, but you can physically adjust the amp output so it is not constantly putting out max amps, but this can’t be done remotely. You can turn on and off remotely, but not adjust.

__mike__ said:

Hello!

I'm adding a 12V system to my van, and the best choice appears to be the 280Ah lifepo4 DIY batteries.

Realistically, I only need about 20 to 30Ah at the moment - a fan, some lights, a diesel heater. Down the road, maybe a year or two, I'll be adding higher-draw items such as a fridge or cooler, coffee pot, etc. There will also be many days on which I don't use any power at all.

Is there a downside to having so much excess capacity? If I'm discharging only a tiny bit, and then recharging, is this bad for LFPs? Anything else I should consider?

Many thanks!

Click to expand...

Nobody has brought it up, but I would have thought a diesel heater would completely bust your 20-30Ah energy budget. I don't have any experience with them, but I thought they used 4-5A continuously, after a heftier startup.

Ai4px said:

Full charge or “short charge”. I think you mean you only want to fill the battery to, say 60% SoC on a “short charge” and 100% on a “full charge”? Have I understood the question correctly?

...snip...

So does your BMS have any relay outputs for SoC? Or do you even want to be able to charge to 60% or 70%?

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Yes - a few in this thread recommended keeping the pack at less-than-100% charge, if I understood correctly.

I haven't picked a BMS yet.

I could likely keep the charger turned off, and then just turn it on every so often; that would keep the battery at a less-than-100% state, with occasional full charges? Or use a SoC reading to trigger the charger on/off, but that may be more complex than I require. What do you think?

Horsefly said:

Nobody has brought it up, but I would have thought a diesel heater would completely bust your 20-30Ah energy budget. I don't have any experience with them, but I thought they used 4-5A continuously, after a heftier startup.

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I *think* they draw about 1-2A when running; but I've seen a lot of numbers thrown around. Definitely about 10A when firing up, but that's just for maybe 5 minutes. I realistically only run it for a couple of hours when I use it, so even at 4-5A I'd be ok. I appreciate the looking out, though!

__mike__ said:

Yes - a few in this thread recommended keeping the pack at less-than-100% charge, if I understood correctly.

I haven't picked a BMS yet.

I could likely keep the charger turned off, and then just turn it on every so often; that would keep the battery at a less-than-100% state, with occasional full charges? Or use a SoC reading to trigger the charger on/off, but that may be more complex than I require. What do you think?



I *think* they draw about 1-2A when running; but I've seen a lot of numbers thrown around. Definitely about 10A when firing up, but that's just for maybe 5 minutes. I realistically only run it for a couple of hours when I use it, so even at 4-5A I'd be ok. I appreciate the looking out, though!

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So "short charge" means not 100%.... making sure I understand what you are getting at. So that's easy... just set the charger voltage to 13.2v. I'm using a 48v configuration, but translated to your 12v system I call full 13.8v. The absolute highest voltage would be 14.4. There's really not much extra energy in the range between 13.5 and 13.8v.

Thanks!

__mike__ said:

Thanks!

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I’d like to take a moment to elaborate a little for you. I love analogies. You know when you fill up the gas tank in your car and you pump 10-15 gallons into the tank and suddenly the last little bit fills up the tank and the filler neck? These LFP batteries a like that. You can dump current in to them and their voltage stays at 3.2v per cell…. Next thing you know they are at 3.42v per cell and rising fast. Just like your gas tank doesn’t hold much volume once you get into the filler neck, so these batteries really don’t hold much energy above 3.4v.

Here’s an example… my 48v battery was 0.16V higher than the other 3 that I was atttaching it to. It’s cells were at 3.46v and the other batteries in the other packs were at 3.45v (16 cells in a pack). When I connected the battery it briefly pulled 25 or 26 amps. Within 30 seconds that extra 0.16v had pulled down to the others’ voltage. Now If my 3 packs had been at 3.1v and the new one had been at 3.2v it could have drawn 50-100 amps for 45 minutes (or longer).

So there’s not much point in charging above 3.45v per cell… there’s just not that much energy in there. Also when you select a BMS set the balance start voltage to 3.45v per cell. When the cells are charging, some of them will “hit the filler neck” before their peers… one of them will rapidly rise past 3.45v to 3.6v. You need to allow your BMS balance feature the time to pull that one cell back to the same voltage as the rest. Once they are all at 3.45v your pack will be close enough to top balanced. Some purists will suggest you charge them once to 3.6v per cell and balance them there.. then they will certainly be balanced at 3.45v Per cell.

Ai4px said:

I’d like to take a moment to elaborate a little for you....

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This is very helpful! Thanks so much for taking the time.