Trailer battery charging

[sfpcservice]

Ok, this is probably more of a camping question but since I have to tow the trailer here it goes:

When I draw my trailer battery down and need to charge it while camping, how much of a charge will the sienna put out at idle if I charge the trailer batter directly from jumper cables? I have heard since I was a kid that cars don't charge at idle, but I'm not sure if that's true?

[brian_bp]

Maybe cars didn't charge at idle, but if you put a voltmeter on the battery (or into an accessory socket) of a modern vehicle at idle I think you'll see lots of voltage. Typical alternators seem to put out their full rated current into a normal operating voltage load at only 2000 rpm or so, and still have a substantial fraction of their current capacity down to idle speeds. Since the Sienna alternator is rated for something like 150 amps (at full speed), even at 700 rpm I'm pretty sure it can put out more than enough to charge the trailer battery at a reasonable rate (only one tenth of that 150 amps would be a decent rate) if the Sienna's charging system decides to run at charging voltage.

I have not had charging system problems with our Sienna so I have not investigated, but when I had a problem with my Ford Focus I discovered that the engine control computer and the charging system controller communicated to decide on the voltage to deliver, based on factors such as whether or not the engine was warmed up. I really don't know under what conditions the Sienna will decide to switch from a maintenance voltage of not much more than 12V (which won't charge the trailer battery very quickly) to the full-blast final charge rate of over 14V, or to a voltage in between.

Remember that when driving or only idling, any vehicle's charging system is not configured to optimally charge an auxiliary battery. This sounds crazy, but if you plug an inverter into the idling Sienna, and an advanced battery charger into the inverter, that charger will probably charge the trailer's battery better than a direct connection to the Sienna's 12V system. No, I have not done this, but others have (in another forum).

If anyone has more specific information or actual test results, I would be interested to hear them. Corrections are welcome.

[datsa noydb]

OK, I have not yet connected my full complement of trailer wiring (only have the lights, so far), but just the other day I had my clamp-on ammeter out, so I put it on the alternator output wire. The van was 'cold', as it had not been run for several hours, but the air temp was in the 90s. It was running at fast idle and I had the A/C on, both fans on LOW. The output of the alternator at that time was 61 amps. I moved the meter over to the battery + wire and saw that only 11 of that was going to the battery.

I will try that again in a couple of days, after the van has warmed up and come down to a normal, hot idle of about 600 RPM.

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[brian_bp]

It was running at fast idle and I had the A/C on, both fans on LOW. The output of the alternator at that time was 61 amps. I moved the meter over to the battery + wire and saw that only 11 of that was going to the battery.

Well we now know that at least 61 amps is available from the alternator at fast idle.

I would measure the voltage, because that is the basis for the regulator's behaviour. If you add more load to the system (turn on something else or connect a not-fully-charged battery), the regulator should attempt to maintain the same voltage, and additional current will flow to the added device/battery. If the voltage at idle is 14+ volts, the trailer battery is going to get charged at least to some extent; if it is down under 13V, then the fans run fine but that trailer battery isn't going to get much added to it.

[brian_bp]

... just the other day I had my clamp-on ammeter out...

Nice tool. I have a clamp-on ammeter for AC, which is cheap because it is just an induction pickup. Induction doesn't work for DC, so this must be a Hall-effect or similar design. With the high currents involved, a typical Volt-Ohm-Milliammeter just isn't up to the task.

[datsa noydb]

Not sure about the technology behind it, but it's available at Radio Shack for $80.

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[brian_bp]

Not sure about the technology behind it, but it's available at Radio Shack for $80.

An excellent deal for a clamp-on DC ammeter. The web page and even the manual say nothing about the technology, but I'm sure it uses a Hall-effect sensor to detect magnetic field resulting from current flow.

Usually these are a lot more expensive; the cheapest at Home Depot, for instance, is $249. We don't have Radio Shack in Canada anymore (replaced by The Source, which doesn't have a clamp-on meter), but there are some other reasonably-priced units out there: Sears appears to have a Craftsman Model# 82369 for only CAD$66.25.

Clamp-on would be really nice for analysing the charging system, because it means no wires need to be disconnected, no large currents need to be handled by meter test leads, and the meter doesn't affect the charging system operation.

[datsa noydb]

Just looked at the Craftsman meter. Not bad, and it's even cheaper than the Radio Shack model.

Not that it matters all that much, but it will measure "only" 400 amps, the RS meter will do 800.
Other than that, they seem to have similar capabilities and specs.

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