Hi John,
Without going to far down this hole, I like Bob's answer. The charge controller should limit the current to an amount that the charger and the batteries can both handle. You are probably pretty familiar with the concept, the alternator in your car has a built in charge controller (voltage regulator). Older cars have external regulators as do many marine charging systems.
If you look at my 160Ah lithium battery bank, it is rated to accept more than 25kW of charging current (480A @ 54V), if available. But my shore charger maxes out around 1.5kW (28A @ 54V) so that it will run on a 15A 115VAC circuit. The battery charger's circuitry (controller) limits the available current and voltage based on the battery's state of charge, i.e. it runs full bore until the battery is almost full.
The botom line is that you are correct in stating that you might be overthinking this and the charge controller should prevent anything from frying. Now, if you were going to design your own charge controller, then you would need to understand more of the details...
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Bob Caulk <bob_caulk@...> wrote:
>
> John-
> The simple answer to your question is that, even though the load (battery) is
> the same, the current is limited in the charger.
>
> Most batteries, when discharged, will accept a very large amount of current, so
> the current needs to be limited by the circuitry inside the charger.
> There are a number of ways this is done. For simplicity you can think of it as
> a variable resistor in the charger. In actuality, it is quite a bit more
> complicated.
>
> Bob
>
> Â
>
>
>
> ________________________________
> From: John Green <v_2jgree@...>
> To: electricboats@yahoogroups.com
> Sent: Mon, May 23, 2011 12:45:20 PM
> Subject: [Electric Boats] I would like charging amps explained pls.
>
> Â
> Hi all,
> After having controllers explained a while back, and now agreeing that
> they are the cats meow, the idea of pulsing dc instead of lower volt dc
> to save battery drain is genius, so, it is time for attempting to
> understand more things.
>
> I have another major thing that I just cannot get my head around, and it
> is the amps used when charging.
> I understand amps when running something, and that a starter motor from
> a car, for instance, has thicker wire in the windings, and will
> therefore draw more amps than a heater blower motor or similar.
> This seems to mean that the greater the load, the more amps get drawn,
> and the shorter the battery charge will last. Simple.
>
> Now, what about when charging?
> I know that some car battery chargers have a select switch, for, say, a
> 2 amp trickle charge, or a ten amp regular charge.
> Yet the battery is the same, so the same rules as for running something
> FROM a battery, do not apply to when a battery is being charged. The
> load is the same battery, so how the heck can the amps change? I can
> understand that when the battery gets almost fully charged, then the
> amps will decrease, as there is less load drawing the amps. And I can
> understand that if a battery is extremely low in charge, it might tend
> to suck more amps, so the charger meter will show a greater amount of
> amps, then slowly get less amps drawing as the battery condition
> improves. But my basic question is, how do the charging amps change at
> the flick of a switch?
>
> My reason for wanting to understand this is that I am collecting parts
> to build a wind generator that will work in conjunction with the
> electric motor and battery etc (that I am currently also gathering parts
> for) in a boat. I know that if I rewind the generator armature with
> thinner wire to get more turns onto it, that the power created is
> proportional to the number of turns, assuming the rpm is the same. That
> is, double the turns, double the volts. (But less amps are possible due
> to the thinner wire).
> The thing is, how thin can I go with the windings before the charging
> load (that I cannot understand) is too great for the generator, and it
> overheats? My understanding is that I need around 15 volts as a minimum
> figure, then this would feed a charge controller, which then feeds about
> 13.7 volts to the 12v battery.
> Or am I overthinking here, (like I have been known to before!) and the
> charge controller will prevent anything frying?
> Simple explanations appreciated.
>
> Thanks,
> John
>
> --
> http://www.fastmail.fm - A fast, anti-spam email service.
>
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