Is Battery Charging Better in Series or Parallel?

We can wire our batteries in series or parallel as I have written about in my previous article.

Now when you want to charge your batteries, you might think that you can charge it faster if you put it in a series or parallel configuration.

Does Series or Parallel Charge Faster?

The short answer is no. Batteries don’t charge faster when you put them in series or parallel. They still have to absorb the full energy capacity (watt-hours).

BUT

That’s if your charger is limited to a certain voltage. If your battery charger is limited to 12 volts, then you should wire your batteries in parallel (if you have two 12V batteries).

If your charger has a variable voltage, 12 or 24 volts, then it’s better to charge them in series. Let me explain with an example.

You have two batteries, each 12Volts 100Ah and you have a charger that can charge at 12V or 24V and is rated for 10Amps.

If you parallel connect, then you have a 12 volt 200Ah battery. You need to have your charger set at 12V.

Parallel connection: P=12V x 10A = 120Watts

If you connect your batteries in series, you will have a 24V 100Ah battery. You need to have your charger set at 24V.

Series connection: P=24V x 10A = 240Watts

Since battery chargers are limited by current, we can see that the 12V parallel connection needs double the time to charge than the series connection.

If you have two batteries and you can make a 24V configuration, you should charge your batteries in series.

Does Series or Parallel Give More Power?

Wiring a lithium battery in series or parallel doesn’t give you more power. However, when we look at the C-rate of the battery we can optimize for the current draw.

If you have a 100ah lead-acid battery with a C-rate of 0.2, then it’s best for the battery to charge at 20amps max. This is to prolong the battery lifespan.

If you charge at a higher current, then the lifespan will decrease and you can store less energy in it because of heat generation.

Let’s say you have the same 12V 100Ah batteries and a 40A charger, then you should wire your batteries in parallel.

Let me explain: connecting two batteries in parallel will increase the Ah (capacity) of the battery, while the voltage stays the same. You now have a 12V 200Ah battery. If the C-rate of that battery is 0.2, then you can charge and discharge that battery efficiently at 40Amps. Because 200Ah x 0.2C-rate = 40 Amps. Charging the battery with 40Amps is going to be better to keep the battery healthy over many years.

If you connect the same batteries in series, then you will have a 24V 100Ah battery. 100Ah x 0.2C-rate = 20 Amps. Charging the battery with the same 40Amps charger will damage the battery because the battery is rated at only 20 Amps charge and discharge current.

Take a look at my video about C-rate:

A Warning About Wiring in Parallel

When you wire your batteries in parallel, you should not use one battery as the main battery. You should wire in a way that the first battery has the positive or negative and the last battery has positive or negative lead. As shown in the following image.

If you do not wire the batteries like this, the battery that is furthest away will not be cycled. That means the load will get pulled from the first battery. This results in the first battery having to perform more work, thus reducing the lifespan of the first battery.

Read my other article about wiring batteries in series or parallel for more information.

Conclusion

It is faster to charge your battery in series if you have a 12/24 selectable power supply.

Never connect a charged and discharged battery. Make sure they are at the same voltage or state of charge (SOC) before connecting them.

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