Lithium vs Lead acid: a charge profile primer

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How lead acid and lithium batteries charge

Recently I was asked if lithium batteries( lfp) accept more voltage from solar panels than lead acid batteries (fla). The short answer is No. But, they do accept more amps! You see as your batteries charge the peak voltage limit is hit and then amperage begins to taper.

The formula you need to remember is voltage X amperage = watts. In battery charging amperage is key.
The formula you need to remember is voltage X amperage = watts. In battery charging amperage is key.
Amperage is the key

A is the voltage my batteries are at right now. I just took the screen shot.

Under that is the voltage my batteries will get to. It should read 14.5 but the batteries are cold.

B is the amperage going into my batteries. This is the most important number.

C, the watts: Its Voltage (A) times Amperage (B) equals watts (C).

So lithium in the (lfp) configuration charges more efficiently than flooded lead acid (fla) because it doesn’t have the resistance that lead has. So my fla pack will hit 14.5 today probably around 80 amps charging. Once that happens (around 10 a.m.) the amperage begins to drop so that the voltage doesn’t rise any further. My batteries are accepting the maximum amount of power they can handle. After 3 hours the voltage will drop from 14.5 to 13.5 or float. And amperage will drop to 0 until the float limit is hit and then will slowly climb. Mostly due to loads like lights, the fridge etc. The batteries are fully charged.

Lithium on the other hand, the amperage will stay at 80 as the voltage will not rise as quickly, no resistance. Once the voltage does rise to 14.2 (a good voltage peak for lithium) the amperage will quickly taper to 0. That’s because the batteries are now full. The solar controller will drop to a float voltage of 13.2.

Advantage lithium

This is one of the advantages of lithium. The ease with which they accept a charge. You get maximum watts out of your solar panels for a longer period of time. And your “absorb” period is much shorter. If you were charging off a generator/charger combo this would save a lot of gas. A lot.

This is tricky. Just remember volts and amps are linked. And it’s the amps going in that counts the most.

The charge voltage tells you the state of your batteries (how full they are). In fla you have to look at amps as well. In lithium it’s clearer where your batteries are at just based on voltage.

An example of this is my batteries. By experience I know that when they near fully charged the voltage will still be 14.5 but the amperage will have dropped to 10. But if they are 20% depleted, the amperage required to get to 14.5 can be 80 amps or more.

I’ve simplified things a bit, but I hope this explanation helps you understand better the relationship between volts, amps and battery state of charge (SOC).

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