Car battery chargers Ah spec

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MCH170

Joined May 13, 2021
10
I was browsing amazon for car battery chargers for a project and most chargers I came across had battery Ah specs (eg. 12Ah-100Ah) which I take refers to the maximum capacity of battery they can charge. My question is:
a) Why is there such a specification on chargers? What's stopping them from charging larger (or smaller) size batteries?
b) If I have 2 12V 100Ah car batteries in parallel and I want to charge them, can I use 2 of these chargers in parallel or would that cause problems with the chargers detecting when a battery is fully charged?

Thanks for your time.
 

AlbertHall

Joined Jun 4, 2014
11,962
This is related to the charging current the charger provides. A small battery would be damaged by too much charge current, and a large battery would take a looong time tto chrge with a low current.

Do not connect smart chargers in parallel - separate the batteries and charge them individually.
 

dl324

Joined Mar 30, 2015
14,332
I came across had battery Ah specs (eg. 12Ah-100Ah) which I take refers to the maximum capacity of battery they can charge.

a) Why is there such a specification on chargers?
It's not very meaningful. What's more important is the maximum charging current. That will give you some indication of how long it might take to charge a battery.

That assumes you know how many Ah a battery needs to be fully charged. So it's a moot point because you won't know that.
What's stopping them from charging larger (or smaller) size batteries?
Nothing. The battery will take what it wants. If the charger can't provide that, it'll take what it can get.

What's more important is that the charger is appropriate for the battery chemistry. For instance, SLA and wet lead acid take different charging algorithms.

I have a battery maintainer that says it can output 4A max. I've used it to charge car batteries that were dead. It could take a couple days, but the maintainer also has a restore feature.
b) If I have 2 12V 100Ah car batteries in parallel and I want to charge them, can I use 2 of these chargers in parallel or would that cause problems with the chargers detecting when a battery is fully charged?
You shouldn't charge batteries in parallel or put chargers in parallel.
 

MisterBill2

Joined Jan 23, 2018
11,624
Certainly post #6 is correct. A wet cell charger made for bigger car batteries will damage one of those 7AH gell cell batteries by overheating, in addition to outgassing. And it will explode a 12 volt NiCad cell pack. The six-volt setting will partly recahrge a gell cell battery, and it will also eventually recover some (stone dead) 12 volt car batteries that draw way too much charging current in the 12 volt switch position.
 
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