# Is charging a small battery bank possible using a battery charger and a large battery in series?

What would likely happen if someone wanted to charge a series battery bank of say 36 volts total but they only had a 24V charger? Could they put that charger in series with a 12V battery and charge the bank that way? For example, imagine they had three 12V 7.2Ah AGM batteries in series like the kind used for a childs electric scooter. The regular charger for that scooter broke and they have to use a makeshift charger but the only other charger available is a 24V but luckily with an AGM mode setting. Knowing this is not enough voltage to charge a 36V bank and not wanting to charge in groups of 2 batteries (1+2, then 2+3, then 1+3), someone decides to just take a larger 12V 50Ah fully charged battery and put it in series with the 24V charger.

Question: will this work and assuming the charger is set for 2A maximum charge current, will that still be maintained with the addition of the series battery which would bring the 24V charger up to 36V? Since the battery and the charger do not share a common electrical ground, it seems they would effectively be isolated from each other with no bad interactions.

• Sounds like a very bad idea to me. – Brian Drummond Sep 21 '16 at 9:39
• @BrianDrummond - Why is it a bad idea? Which is the "bad" part? If the 24V charger controls the charge voltage so that only 2A can flow when charging a 24V (nominal) battery bank, then by adding 12V more on the donor side and 12V more on the receiving side should keep things in balance. For example, suppose the 24V charger is indeed set in 2A mode and puts out 26V to get the 2A flowing into a medium discharged 24V battery bank. Charging is then immediately interrupted and a single 12V battery is added to the donor side and another one to the receiving side. Seems like it should work. – David Sep 21 '16 at 11:35