Charging arrangement for an AGM battery

[srp]

Just about to install my new AGM battery. It's the only one on board, and is for engine starting (1gm10), running the echo sounder and one led cabin light.
1. Should I fit some sort of regulator to prevent the alternator charging it at too high a rate, or will the alternator just adjust its' output automatically?
2. When I take the battery out for the winter, do I need to buy a proper AGM-type charger to maintain it, or will a cheap Halfords charger do the job.
I don't really want to spend a fortune on what is really a very basic installation, but I don't want to ruin a new AGM battery through ignorance, so if anyone can recommend particular products I'd be grateful.

 

[alan006]

First of all I have to admit I've never had an Absorbed Glass Mat battery so any comments here are based on what I've read about them together with a bit of guess work.
AGM's are very tolerant of deep cycling so my guess is if you use a "cheap Halfords charger" you are unlikely to damage it. The danger seems more likely to come from overcharging. I'd be interested to know what others say about the alternator but again I suspect that overcharging is the potentially biggest danger to the battery.

 

[srp]

I probably didn't make it clear in my first post, but you're right, it's overcharging that I'm concerned with, since AGM's don't vent like an ordinary lead-acid battery.

 

[pvb]

It\'s not a problem...

A standard alternator won't damage your AGM battery. An ordinary charger won't damage it either.

 

[William_H]

Re: It\'s not a problem...

If you want to be really safe on the charging current...and the current in worst case scenario of very flat battery is limited to what the alternator will supply...
Then the answer is to put resistance in the cable from the alternator to the battery +ve. A .1 ohm resistor will drop 1 volt at 10 amps so limiting the current by reducing the charging voltage. Note this will not reduce the voltage the battery is charged at because when the current falls so does the voltage drop.
So a resistance of about .05 ohm will give some protection. ie dropping 1 volt at 20 amps. This can be acheived by buying a .05 ohm resistor of rating 20 watts (if you can find one) Or by buying 5 .1 ohm resistors (5 watts) and wiring in paralell.
Or you can make your own by puting into the circuit about 12 inches of 1x19 SS rigging wire 3/16 inch diameter. If you can swage a loop on each end it will facilitate bolting a terminal to it.
Use an ampmeter then to determine how much current is being limited.
You will need a large load on your battery like a couple of 50 watt light globes. With the engine charging, resistor in circuit and light globes connected you bypass the resistor to see how much current is being limited. Or by measuring the voltage drop across the resistor. About .5 to 1 volt should be enough.

However after all that I wouldn't worry charge current should be OK as you probably will never discharge the battery anyway.

For charging at home yes use a crude charger for limited periods and often through the winter. You need to know the current your charger is actually puting in. A .1 ohm resistor in series will give .1 volt across it for every amp. Assuming you have a 50AH battery then give it 10 AH every 2 weeks. aprox.

good luck (and don't think about winter yet) olewill