Connecting a second lithium battery

[Roger S]

I have a 230a lithium liesure battery which has gone flat, we will soon be going on a 7 hour drive so it will get a charge on the journey, I would obviously prefer to charge it by mains before but cannot. I have a new 2nd matching battery which I am charging to full capacity before I connect it, my question is when driving the van with the 2nd battery connected in parrallel I assume the 1st battery ( the flat one ) will take the charge ? and secondly we will have enough capacity in the new battery for this trip which I assume will be fine for the habitation side even though the first battery will not be near to being full ?

I will charge the batteries on a mains line after this trip to balance them.

Thanks Roger

 

[bigtwin]

In the first instance, I would simply swop them until such time as you can fully charge them both and have time to install them properly.

By properly, I mean using a busbar to ensure that the charge path lengths are equal. This isn’t so critical with lead acid batteries but is more so with LiFePO4.

Ian

 

[autorouter]

If you connect a charged battery to a partly discharged battery, charge will be transferred to the partly charged battery until it is about the same charge as the previously fully charged one. You should end up with a pair of batteries that are both at about 50%. Then when you are driving, the pair of batteries will be charged as if they were one big battery.

Because the charge states were not exactly equalised on connection, one of the batteries may remain a bit lower than the other, but that won't have any effect short-term, and you can fully charge them separately when you get back, to get them both exactly equal.

Then connect them as bigtwin says - lithium are quite fussy about unequal voltage drops on the charging wires.

 

[AdrianChen]

In the first instance, I would simply swop them until such time as you can fully charge them both and have time to install them properly.

By properly, I mean using a busbar to ensure that the charge path lengths are equal. This isn’t so critical with lead acid batteries but is more so with LiFePO4.

Ian

I presume the BMS will limit the current when the batteries are initially paralleled, or will it decide that it's overcurrent and switch off?

 

[bigtwin]

I presume the BMS will limit the current when the batteries are initially paralleled, or will it decide that it's overcurrent and switch off?

The BMS can’t actually limit the current other than by disconnecting the charge source (the other battery) on over-current.

Whether the current flowing between the batteries would be high enough to trigger over-current protection is hard to say though. The usual recommendation for connecting two LiFePO4 batteries together is to ensure that both are fully charged prior to doing so; this minimises any ‘balancing’ required.

Ian

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[Roger S]

That does inconviently make sense

 

[Raul]

A flat Li battery and a fully charged Li battery should never be paralleled until voltage on both are very close to each other. Its voltage that moves the amps.

 

[Roger S]

Okay thank you, I will as suggested by bigtwin swap them.

 

[autorouter]

A flat Li battery and a fully charged Li battery should never be paralleled until voltage on both are very close to each other. Its voltage that moves the amps.

If I had that problem, I would connect the two batteries with a resistor to limit the amps. First connect the two negatives together. Then connect a resistor between the two positives.

A suitable resistor could be a bulb, such as an old-style filament-type headlight bulb or even a 21W indicator light bulb. The bulb may glow a bit for a short time, then it will go out and the amps will be transferred at a low speed. You can measure the voltage difference easily with a meter. After a few hours the voltages will be nearly equal, and then they can be connected directly.

 

[Raul]

You can buy cheap ceramic resistors for this purpose, but how many times you gonna have this scenario? I just clip a charger on the low one for few hrs, then I hook them up with 35mm2 spare battery links.
I actually used the headlight bulbs on first balance, manually helping the balancer on the high picking cells. I got two bulbs rigged up for this purpose. Good shout to use them as a resistor, haven't tough of that.

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[LithiumConvert]

By properly, I mean using a busbar to ensure that the charge path lengths are equal. This isn’t so critical with lead acid batteries but is more so with LiFePO4.

I don't understand why.

I have two LiFePO4 battery systems, each with multiple batteries of unequal capacities connected with random length wires of unequal diameters. They have been like that for years and seem to work with no problems.

 

[fishplug]

I hadn't heard of the 'busbar' charging method for parallel setups but found this video really helpful in understanding it.



(skip to 10:02 if you don't want to see the other methods)

Your mismatched setup will still work, it just won't be spreading the load / charge evenly so you'll be wearing one battery out faster than if you went for a more regimented setup. The more batteries you have the more this becomes important.

 

[LithiumConvert]

That battery is still designed to last ten years (as it would if was the only battery)

and the others will (by implication) last longer.

That will do me.

 

[Tombola]

Okay thank you, I will as suggested by bigtwin swap them.

Thats the best way.

I don't understand why.

I have two LiFePO4 battery systems, each with multiple batteries of unequal capacities connected with random length wires of unequal diameters. They have been like that for years and seem to work with no problems.

Is it, do you not find any charge differences ?

I added 2 x 200ah a while back and discovered over time that one would always charge less than the other, in the end it got on my nerves and when I dismantled some of the cables including the solar, were a little longer to one battery, I made them equal and all been well since.

 

[bigtwin]

I hadn't heard of the 'busbar' charging method for parallel setups but found this video really helpful in understanding it.

He states that the black wire to the busbar needs to be the same length as the red; that isn't the case and neither do the black battery wires to the busbar need to be the same lengths as the red battery wires.
It’s just a circuit, it doesn't matter if, for example, the positive and negative are both 0.5 m or if the positive is 0.9 m and the negative is 0.1m. In both cases the circuit carries 1m of cable resistance.

It does matter that all battery positives (to the busbar) are equal and all negatives (to the busbar) are equal (to ensure an equal charge/discharge distribution between batteries). The length of charge/discharge wires on the feed to the busbar are immaterial (notwithstanding the normal good practice requirements).

Ian

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[bigtwin]

Is it, do you not find any charge differentials.

Some folks may not be aware of the issue and others are not bothered by the situation.

Ian

 

[MichaelT]

He states that the black wire to the busbar needs to be the same length as the red; that isn't the case and neither do the black battery wires to the busbar need to be the same lengths as the red battery wires.
It’s just a circuit, it doesn't matter if, for example, the positive and negative are both 0.5 m or if the positive is 0.9 m and the negative is 0.1m. In both cases the circuit carries 1m of cable resistance.

It does matter that all battery positives (to the busbar) are equal and all negatives (to the busbar) are equal (to ensure an equal charge/discharge distribution between batteries). The length of charge/discharge wires on the feed to the busbar are immaterial (notwithstanding the normal good practice requirements).

Ian

I fitted my unequal capacity and different makes like this (did have equal lengths of 500mm 50mm2 on all cables) and charged new one to 86% before adding in. Let solar do it's job and the batteries just about charge and discharge equally.

I never hook up and only use solar and B2B for charging.

 

[bigtwin]

I fitted my unequal capacity and different makes like this (did have equal lengths of 500mm 50mm2 on all cables)

And they will, quite happily, charge/discharge in direct proportion to their relative capacities.

Ian