cheap battery charger?

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ben85

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I'd go with the 20A charger - the 40A charger represents more charging power than an ordinary AGM battery can handle (max rate = C/3 = 33A in your battery's case). A spiral wound AGM would take C/2 but the biggest is 75Ah (Optima D31A) so the 40A charger is still a little large for that.

It's nice to think that you can charge your battery bank up in an hour or so, but charging batteries at too high a rate will cause the batteries to overheat and possibly start venting hydrogen. Even though a battery is marked "Sealed", it isn't completely - they're actually called "VRLA" (valve regulated lead acid) which means that once a certain pressure is reached inside, they'll vent regardless. Charging at too high a rate can cause this.

20A at even 15V is just 300W, so your 800W generator can easily do it. If it's a cheap 2-stroke Bunnings special (or similar), whack a 150W flood light on it before you connect the charger. Output voltages on those 2-stroke gennies vary from 150V up through to near 400V. Even a switchmode power supply is going to have trouble handling that, but the gennies settle once they have a light load on them.
 
The faster you charge a battery, the shorter the life. It is a basic law of batteries. Look up the chart that shows Rate of Charge Vs Life Cycles.

My rule of thumb is that you need to spend as much on the charger as the battery if you want a good life.
 

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