# 12V Battery Charger that runs of 12V DC?

Discussion in 'Power Electronics' started by jjmalove, Oct 18, 2016.

1. ### jjmalove Thread Starter Member

Jul 6, 2016
39
1
Hello,

I'm trying to improve a flaw in an ongoing work project. Here is the basics:

We have a system with an AGM 12V Battery. The battery powers some 12V DC/DC converters (which cleans up the variable voltage into a clean 12V and filters out any crap), which then power a mixture of circuit boards.

We currently have a setup in place to either charge the AGM battery with 110V AC Input to a COTS battery charger, or we can charge the AGM battery by being tied into a car's alternator. The way we successful tie into the alternator is a little tricky but sufficient to say we have it working and we can connect and disconnect at will through a relay with software or a manual action.

But I don't like how the alternator is charging the battery. My AGM battery says it wants a 3-5 amp charge rate. The AC battery charger gives it 5-6, and trickles lower as it gets closer to max.

The alternator, however, gives it very little amps when the battery is at 90%+, between 0.5-1.0A, and gives it too many amps when the battery is low charge, 10+ Amps. I'm worried this is going to either quickly break the battery at some point, or degrade its life rapidly.

So I think a better solution looks something like this:

I will continue to have these inputs
110V AC Input
or
12V-14.4V~ DC Input (Alternator)

But now I either include a DC to AC Inverter that steps the alternator 12V up to 110V, then use that to power the battery charger, or I have two battery chargers in the system, the first one that runs off 110V AC and the second one that runs off a variable DC Input say 9-16V to charge the battery.

The systems working as is but I'm just trying to think of a better answer for this part. Anyone have any thoughts and any idea if you can even get a DC powered battery charger?

Thanks!

Edit: Oh one more thing. We are only ever tied into the alternator when the car is running and has been running for 15 seconds. So we never have to worry about the cranking action of starting the car affecting us.

2. ### tranzz4md Member

Apr 10, 2015
152
28
I think you're making your own trouble. Of course the automotive alternator will have that charge characteristic; it should. Your AC powered, plug-in charger seems good too. Both have good trickle rates for high charge levels.

Have a look at this:
http://batteryuniversity.com/learn/article/absorbent_glass_mat_agm

Fuss all you want. Eventually it will die. In fact, it's designed for a certain life, and exceeding that isn't likely or wise to count on. Batteries are chemical devices and their life isn't determined by nuances in charges.

3. ### tcmtech Well-Known Member

Nov 4, 2013
2,104
1,747
I don't consider any battery pulling as much as 10x ist Ah rating for the first minute or two from a constant voltage source to be an excessive beginning charge rate.

If it ws my project I wouldn't worry about it. There are numerous other factors in the system that will bring you battery to an earlier than optimal death than what higher than optimal current initial charge events ever will.

4. ### jjmalove Thread Starter Member

Jul 6, 2016
39
1
Ok great guys, thanks!