Charging one battery from another using a Solar charge controller?

[MattiFin]

The stationary battery is a 8s LFP 20-28.8V 20Ah.
The battery that will be connected and disconnected, frequently a 12s 21-28.8V 280Ah lead acid battery in a electric forklift.

This is one of the scenarios that wont work. Full 8S LFP is 3.3 to 3.2 volts per cell --> 26.4v for full 8S LFP under discharge.
12S Lead-acid needs 26.4v charge voltage straight away when empty. You might get the lead acid from 0% to 5% charge but thats pretty much it.

 

[Karottemkopf]

And quite often same-nominal voltage batteries with similar chemistry don't charge at all if connected parallei. Or charging is extremely slow.
Required charge voltage above 10% charge level is higher than voltage of the full battery and charging won't happen or is extremely slow

https://www.custommarineproducts.com/uploads/1/4/3/7/1437708/published/lifepo4-discharge-curves_2.png

Lead-acid is even worse, empty battery jumps above 13 volts almost straight away and full battery under load is only 12.8v

So what do you think will happen, the battery voltages will not equalize even over a small resistor in a reasonable amount of time ?
Just connect them in parallel directly seems a bit risky if there is a 2V delta or more

 

[Karottemkopf]

This is one of the scenarios that wont work. Full 8S LFP is 3.3 to 3.2 volts per cell --> 26.4v for full 8S LFP under discharge.
12S Lead-acid needs 26.4v charge voltage straight away when empty. You might get the lead acid from 0% to 5% charge but thats pretty much it.

Ok
That the LPF battery would not charge the lead acid battery significantly on its own is not really a problem in my scenario.

The question is will a solar charge controller connected to them in parallel will charge both of them?

 

[Vigo]

Efficiency is difficult to compare because the two conversion have a constant power loss.
Compared to connecting them in parallel over a small resistance only waste power during voltage equalization, and then I can connect them directly and no more power will be lost.

Well going back to what mattifin said, you are not going to get 'true' equalization. The charge transfer will be largest when first connected, but it will quickly drop until nothing is useful is happening, long before the batteries are truly 'equal'. It is a very useful thing for, say, engine starting as in the case of car 'jumper cables'. Even with small jumper cables, you can still transfer enough charge from a 'full' battery to an 'empty' one to crank an engine so the thing can resume charging itself. Or if you had one battery which was 'stuck off' or asleep from discharge, you could get it to 'wake up' so you could resume charging it normally.

 

[Karottemkopf]

Well going back to what mattifin said, you are not going to get 'true' equalization. The charge transfer will be largest when first connected, but it will quickly drop until nothing is useful is happening, long before the batteries are truly 'equal'. It is a very useful thing for, say, engine starting as in the case of car 'jumper cables'. Even with small jumper cables, you can still transfer enough charge from a 'full' battery to an 'empty' one to crank an engine so the thing can resume charging itself. Or if you had one battery which was 'stuck off' or asleep from discharge, you could get it to 'wake up' so you could resume charging it normally.

No, I understand that they're not going to be equalizing quickly or completely at all, but that's not my goal.
It is to be able to charge them from the same SCC and use them both as energy storage and to power the inverter, which I'm quite sure it will do.

What I am unsure about is how large the currents would be initially when connecting in parallel if the there is a large difference in the Soc of the battery and if putting a power resistor in the middle to limit that current is required.

 

[Vigo]

If you have a circuit breaker of a 'safe' amp rating for the batteries and the conductors you will use to connect the batteries, i would just try it with that and see what happens.

Another thing you could do is use an incandescent lamp between the two. The resistance of an incandescent lamp filament is low when it is cool, but goes up rapidly as it warms (which limits circuit current). It is essentially a homebrew 'PTC thermistor' with a visual indicator function.

 

[sunshine_eggo]

No, I understand that they're not going to be equalizing quickly or completely at all, but that's not my goal.
It is to be able to charge them from the same SCC and use them both as energy storage and to power the inverter, which I'm quite sure it will do.

What I am unsure about is how large the currents would be initially when connecting in parallel if the there is a large difference in the Soc of the battery and if putting a power resistor in the middle to limit that current is required.

 

[MattiFin]

The question is will a solar charge controller connected to them in parallel will charge both of them?

It works somewhat ok, you get reduced lifetime from one of the battery sets.
Set charge voltage to 3.45v per cell and you get 13.8v
That would be perfect stand-by charging voltage for lead-acid but leading to very slow charge/and/or undercharge.
Lifepo4 can tolerate bulk stage charge voltage to 14.4v with somewhat reduced lifetime and that should be quite satisfactory for the lead-acid also.

So it is not perfect solution but not total disaster either.

 

[Karottemkopf]

Yes I watched this too, made me think that some power resistor might be a good idea as the voltage delta could be much lager in my case

 

[sunshine_eggo]

Yes I watched this too, made me think that some power resistor might be a good idea as the voltage delta could be much lager in my case

How would it be "much larger?" 5-6V? You can minimize that by timing the connection.

 

[Karottemkopf]

If you have a circuit breaker of a 'safe' amp rating for the batteries and the conductors you will use to connect the batteries, i would just try it with that and see what happens.

Another thing you could do is use an incandescent lamp between the two. The resistance of an incandescent lamp filament is low when it is cool, but goes up rapidly as it warms (which limits circuit current). It is essentially a homebrew 'PTC thermistor' with a visual indicator function.

Circuit breakers or fuses what would be better?

 

[Karottemkopf]

How would it be "much larger?" 5-6V? You can minimize that by timing the connection.

If the forklift comes back empty at the end of a day and the stationary batter is fully charged

 

[sunshine_eggo]

If the forklift comes back empty at the end of a day and the stationary batter is fully charged

So, 5-6V... maybe as high as 8-9V if you want to connect at peak absorption. Your 16S fully charged battery shouldn't be sitting any higher than 54.4V (3.4V/cell).

 

[Karottemkopf]

So, 5-6V... maybe as high as 8-9V if you want to connect at peak absorption. Your 16S fully charged battery shouldn't be sitting any higher than 54.4V (3.4V/cell).

yes 8-9 V is that high i could expect so i thought 0.5 Ohm resistors to limit the max current below to blew 20A. And after the current drop below 1A connect the directly.

 

[MattiFin]

yes 8-9 V is that high i could expect so i thought 0.5 Ohm resistors to limit the max current below to blew 20A. And after the current drop below 1A connect the directly.

You are ok as long as the lifepo4 BMS or fuses are not tripping on overcurrent.
quite tiny resistance (fuses, cables) already limits the current to tolerable levels.
Lead-acid can tolerate 1.5C 1.5*280=over 400 amps) charge current as long as your voltage is reasonable (less than 30 volts, not really issue)

 

[Karottemkopf]

You are ok as long as the lifepo4 BMS or fuses are not tripping on overcurrent.
quite tiny resistance (fuses, cables) already limits the current to tolerable levels.
Lead-acid can tolerate 1.5C 1.5*280=over 400 amps) charge current as long as your voltage is reasonable (less than 30 volts, not really issue)

The BMS overcurrent is what tripping is the only thing that might happen power resistors would prevent that

 

[Vigo]

The thing about resistors is that finding one of the right resistance is easy, but finding one that can dissipate the amount of heat it will be creating without melting becomes much less practical as the numbers go up.

I suspect it would be easier to just use some other sort of device you already have sitting around as this safety feature. If you are determined to use a resistor with low resistance that can dissipate a fairly large amount of heat, 'ballast resistors' for old car ignition systems are very cheap and readily available with prime shipping on Amazon. Many of them have their resistance actually written on their housings, and some are two resistors built into one housing which gives you series/parallel options. I'm not sure the availability of actual large watt rated resistors is as easy, but i can't say i've ever tried.

 

[OffGridInTheCity]

The only boost charge controller I've run across is the MPT7210A - https://www.amazon.com/MPT-7210A-Controller-street-charging-system/dp/B01DP57PYA You can customize the battery charge settings (very cool) and it works (will boost lower voltage to higher battery voltage) but.... it's MPT (not MPPT) and it's limited to 10a. And it used to be $45'ish.

I have one in my DIY solar generator 'in a cooler' and I use it to boost 12v 'portable' panels -> 7s DIY 18650 battery. It's been around for a few years and there are several youtubes on it you get interested.

You could feed in one battery and charge another... but maybe an inverter -> proper-charger (with more than 10a) will give you more options such as powering the charger from a battery/inverter OR just using a wall socket while the grid is working

 

[MattiFin]

The thing about resistors is that finding one of the right resistance is easy, but finding one that can dissipate the amount of heat it will be creating without melting becomes much less practical as the numbers go up.

I suspect it would be easier to just use some other sort of device you already have sitting around as this safety feature. If you are determined to use a resistor with low resistance that can dissipate a fairly large amount of heat, 'ballast resistors' for old car ignition systems are very cheap and readily available with prime shipping on Amazon. Many of them have their resistance actually written on their housings, and some are two resistors built into one housing which gives you series/parallel options. I'm not sure the availability of actual large watt rated resistors is as easy, but i can't say i've ever tried.

You don’t need much resistance if the batteries have similar voltages.
Initial surge current is not long-lasting if allowed to reach reasonably high value.

Lightbulbs are one good option as they increase in resistance as voltage/current goes up and act like sort of constant-current limiter. 10x 55W 12V car headlight bulbs cost 20 bucks or something like that and limit the current to 50amps or so.
Other creative options would be a long piece of wire/cable, 6 feet section of rebar rolled to coil etc

 

[Vigo]

If one is comfortable with being fully in charge of the situation and owning the consequences, even tightening terminal bolts differently to 'manage' the connection as shown in the video will 'work'.. but whatever you do, unless you are 100% confident it cannot become unsafe, i would make a connection that does not require you to unthread anything to abort the operation! Needs to be 'pull-apart' or 'knock-apart with the safety stick'.. And considering the main danger here is heat, i would probably just put on the gloves BEFORE you run into something being too hot to separate with your hands.

It could be taken as being safety-first or over-dramatic.. if you understand the situation well enough you will be in no danger. But sometimes our faith in ourselves is the most dangerous thing.