Charging only when driving, not idling - Ford Transit USA Forum
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post #1 of 14 (permalink) Old 01-10-2019, 09:07:PM Thread Starter
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Charging only when driving, not idling

I've been reading a recent thread on alternator charging the house bank via a solenoid, and my mind wandered a little. I know that drawing high current while idling and the vehicle is not in motion is not a good thing. The alternator is turning slowly and there isn't enough air movement for cooling.


Let's assume for a moment that I'm good with electronics (because I am) and I could use the vehicle speed signal that I'm pretty sure is available under the driver seat. Would there be a considerable advantage to setting up a system whereby your battery charging solenoid is only activated over a certain vehicle speed to ensure that charging happens only while "driving"?
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post #2 of 14 (permalink) Old 01-11-2019, 12:12:AM
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If you're good with electronics, perhaps you could just sense the current from the alternator and trip the charging when it exceeds a threshold.

Just a guess here — maybe using a resistor to convert current to voltage and an OpAmp wired as a comparator that trips a relay when the reference voltage is exceeded?
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post #3 of 14 (permalink) Old 01-11-2019, 06:45:AM Thread Starter
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Current isn't the biggest problem, rather temperature - which is related to current, time, and heat dissipation. I suppose a temperature sensor would work (maybe better) but I prefer to avoid routing wires if I can avoid it.

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post #4 of 14 (permalink) Old 01-11-2019, 07:21:AM
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Sounds like a great project, using an RPM reading to engage a relay would be interesting. The other thing could simply be a manual switch you normally leave on, but turn off when idling for more than a short while.

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post #5 of 14 (permalink) Old 01-11-2019, 07:44:AM
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Originally Posted by asdrew View Post
The other thing could simply be a manual switch you normally leave on, but turn off when idling for more than a short while.
Except then you may have buggy wet-ware to deal with.
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post #6 of 14 (permalink) Old 01-11-2019, 08:25:AM Thread Starter
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I'm not a fan of the manual switch, for the reason that @Travlin astutely pointed out.

The idea of RPM based switching isn't bad, but it means I'd need to use an OBDII PID through a bunch of complicated and unreliable systems. Of course there are other ways to get a tach signal, but none of them as simple as the signal I have ready access to under the seat right next to the battery I'd be tapping into. (Or is there one that I'm unaware of?)

The other reason I'd hesitate to use RPM is that on a cold morning the idle is increased while the engine warms up. I haven't paid too much attention, but it seems to me that this may overlap with some of the normal range while driving in certain gears. It's speculation at this point.

It also brings up the question as to whether Battery to battery (B2B) chargers take this into account. From what I know, they want a certain voltage level to be present for a while before taking high current. I don't think they have a way to discriminate between driving down the road and sitting there idling.

Yet another question is whether there's already an alternator temperature sensor, and if I can gain access to that through the OBDII port. I still don't like the increased complexity, but it would give me the correct information upon which to base a decision whether to draw current or not.
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post #7 of 14 (permalink) Old 01-11-2019, 09:09:AM
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Interesting..I never considered charging while idling could be a problem. In fact, I was thinking of occasionally do this (say 20-30 minutes) to charge house battery. Should I reconsider this? Or is this of short enough duration to not pose a problem?
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post #8 of 14 (permalink) Old 01-11-2019, 09:17:AM
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I think a better approach would be to control the duty cycle. This way you can still charge while idling, and you're helping to control alternator temp at all times. 15 min on, 15min off for example.
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post #9 of 14 (permalink) Old 01-11-2019, 09:52:AM Thread Starter
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I think a better approach would be to control the duty cycle. This way you can still charge while idling, and you're helping to control alternator temp at all times. 15 min on, 15min off for example.
You're just guessing then. If it's an extremely hot day with the A/C going and a depleted battery bank, and idling this strategy may not be sufficient. Likewise, under ideal conditions you're missing out on 50% of the available charging power.

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post #10 of 14 (permalink) Old 01-11-2019, 10:06:AM
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You're just guessing then. If it's an extremely hot day with the A/C going and a depleted battery bank, and idling this strategy may not be sufficient. Likewise, under ideal conditions you're missing out on 50% of the available charging power.
You posed a question regarding battery charging only, not that you needed to power a constant load.
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