Car engine starter inadvertently charges battery?

I'd suggest checking the open-circuit voltage, or the voltage under light load, just to confirm this. I expect you're right, though.

A lead-acid car battery can take a rather amazing amount of recharge current. Some of the "pro" chargers are rated at 40 amperes or more of charging current (and as much as 200 amps of starting current) and can fast-charge a battery in an hour or so.

Even the relatively small charger I have at home will do 15 or 20 amps.

The real question is, what's your battery's condition at the time you establish this hookup? If you need to use a jumpstarter, this implies that your battery's already rather flat, and if so it's almost certainly going to want to swallow a lot of current all at once.

You'd need a more accurate DC ammeter to determine how much current your battery is actually swallowing, at this point. I would not, however, expect that having this much current flow into it, briefly (before you crank the engine) would be likely to harm it.

If you're concerned about this, it wouldn't hurt to set the charger to its 10-amp setting, hook it up, plug it in, and let it "bootstrap" the battery back to a better charge state for a few minutes. Then, switch it to the 50-amp jumpstart position and give 'er a crank.

The risk of a spark-triggered explosion is perhaps a more serious issue. The charging process can release hydrogen from the battery, and a spark upon connection/disconnection could touch it off.

The best way to avoid this would be to avoid a hookup arrangment (with a charger, or when jump-starting) which could create sparks in the vicinity of the battery. The techiques I've been told:

- If you're going to hook up an AC-powered charger or jumpstarter, make the connections to the battery *before* you plug it in. No AC, no voltage, negligible current flow, and thus no spark.

- If you're jump-starting from another vehicle, do *not* make the final connection to *either* battery. First, hook up the black jumper cable to the battery ground post of the vehicle that's providing the jump-juice. Then, hook up the red jumper cable at both ends (right to the battery). Finally, make the black-cable jumper connection for the vehicle which needs the jump... but do *NOT* hook up to the battery. Instead, ground to the engine block, close to the starter. That way, if there's a spark when you make the final connection (as there often is) it's safely away from the battery. After jump-starting, disconnect in the reverse order.

You could certainly do this. However, it would require fairly "stiff" regulation of the supply... some form of switched or linear regulator... and that would add quite a bit of cost to the system. Also, by lowering the voltage supplied by the charger, it would reduce the starter's cranking power... which might be just the wrong thing to do if you're starting under difficult conditions (e.g. in the snow, with a really cold and sluggish engine).

Car batteries routinely take a 100amp or more charge after you start the engine, until the voltage comes up to about 13-14 volts, this is normal. And the engine start mode of battery chargers are meant for a short duration until the engine is started. They usually have a amp meter that gets pegged in the start mode, the voltage will also droop under the starter load.

Its all about cost, since lead acid batteries can handle short high current bursts. Besides, the battery is probably toast if you have to use the start mode anyway.

Cheers

any car battery charger that doesn't produce more than 12V is pretty useless. it takes over 13V to charge a car battery.

even small altenators produce upto 35A to charge the battery

to avoid explosions follow the instructions.

It involves a sack-barrow with another battery on it, and 200A jumper leads, I've seen them see them at used car yards etc

A starter can pull over 500 amps. The battery can handle that amount of current in either direction.

What is in danger is your charger. However, full scale is only 12 amps. Given the gauge and length of the leads, the circuit is probably incapable of more than 40-50 amps, well within the limits of the charger's electronic circuitry. It should be designed to handle a dead short or have clear instructions that you should disconnect it before attempting to start the car.

short

Most likely the meter is way off. Mine does that too, so I tossed on my Amprobe clampon ampmeter and checked--sure enough it was off by about twice!

My experience of a charger with an "engine start" function suggests that they'll only be effective if the battery is only just incapable of starting the engine on its own. If the battery is dead, then you've got Buckley's chance. I had a battery go from being capable of starting the engine on its own, to incapable, even with support from the "engine start" function, in the space of an hour.

A 3 kW device might be able to do it, but would no doubt come with a price to match.

Sylvia.

I've seen DC ammeters that are essentially a compass. It's a permanent magnet inside the case, on a pivot, connected to the pointer. The meter case has a clip, and it just clips over the wire. When I used to install them, we'd calibrate them by moving the #2 wire around until it was "close enough", then RTV it in place.

Cheers! Rich