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

[Mick Abraham]

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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