[RE-wrenches] Off-grid non-inverter battery charger recommendations?

[Larry Crutcher, Starlight Solar Power Systems]

Hi MIck, 

I often see your explanation for absorption charge phase. Consider this: While charging, current is applied without voltage regulation (CC). At the charge set point, PWM control (CV) takes place to regulate voltage. There is no control over current as the battery internal resistance dictates this. For example, if you have a pure DC source at a constant voltage attached to a battery, once the battery reaches that voltage, charge current will naturally start to decrease. 

I find it interesting that many people, including some manufacturers, believe a PV charge controller or AC battery charger is adjusting or tapering the current supplied to a battery when it is the other way around.

Larry Crutcher
Starlight Solar Power Systems
(928) 342-9103



On Oct 15, 2012, at 10:27 AM, Mick Abraham wrote:

Hi, Mechanix~

Bill Dorsett polled the group about recommended battery chargers for connection to a generator. Iota brand is the successor to Todd brand; I believe Iota bought the designs, tooling, etc. and these have appeal because of the relatively low cost per rated amp. They seem generator tolerant.

The Todd design is a "constant voltage" type charger, however--basically an AC to DC converter with a fixed voltage output. In my experience, the "75 amp" Todd type charger will deliver 75 amps only for a relatively short time when first connected to a low battery. As the battery's voltage comes up that presents a sort of "back pressure" which slows the rate of charge. That's not so bad if one has utility power >always on< to drive the charger, but if an engine generator is running...

The inverter/chargers (with which we are all familiar) begin with a "constant current" part of the charge cycle--aka bulk charge. The charger is determined to sock the battery with the amperage we have requested in the setup; I think a pulse width modulation is going on where the charger hums up its voltage in order to keep the amps high even as the back pressure builds. When it's time for the absorption phase of the charge, the logic shifts to a "constant voltage" where the amps are gradually dialed lower in order to keep the battery at the target voltage so it can "absorb". Ignoring the float phase which would come later, this overall charge regimen would be called CC/CV for "constant current/constant voltage". The CV part of that is not much different from what a Todd style charger does; it's just that the Todd design does not precede that phase with a CC "constant current" phase. 

Some free-standing battery chargers have a CC/CV algorithm but those seem to cost as much or more than a comparably rated inverter/charger. Check the Magnum mod-square multi's, Bill, such as their RD series. As others on this List have noted, just the charge function is pretty good and since the inverter circuit is not pure sine, that keeps the cost lower. Built in time delay before it loads up the generator is an added benefit. 

Jolliness,

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675
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