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Old 02-17-2013, 09:51 AM   #1 (permalink)
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Car to car battery charger

Hi,

I've built something that I find very useful and wonder if others might as well.

The garage I store my Lotus doesn't have a nearby power outlet so I've had to deal with ways to keep the battery charged. The garage is at my work and so I can park my daily-driver next to it and use it as a power source. Instead of installing a battery disconnect switch, I built a car to car battery charger based.

Basically the device connects two cars via the 12V cigarette outlets and has an electronic circuit to keep the current within the safe limit of standard outlets. Of course both vehicles need to have 'always on' outlets and fortunately the Elise does.

If there is enough interest in something like this, I could make a group buy on the parts and either sell them as a DIY kit (some soldering required) or I could assembly them and sell them for a bit more.

best,
Brad
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Old 02-17-2013, 10:29 AM   #2 (permalink)
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I had the same problem so I bought one of these:

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Old 02-17-2013, 01:51 PM   #3 (permalink)
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Yea I have one of those jump starter packs too but don't like using it. My battery cover is a pain to remove because the way my audio amp is wired. However the more important reason is using the pack means the battery is already deep-discharged which isn't good for most batteries (deep cycles are more tolerant). Periodically using the car-to-car charger keeps the battery healthy if I don't drive the car frequent enough.
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Old 02-17-2013, 07:32 PM   #4 (permalink)
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So its similar to this?


Black & Decker Tools | Simple Start? Vehicle-To-Vehicle Battery Booster BBC2CB

So the Lotus would be taking a charge from another car's battery? both are off?
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Old 02-17-2013, 09:16 PM   #5 (permalink)
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Kimgt,

Yes the BBC2CB looks fairly similar. Although based on reviews it sounds like the current-limiting circuitry isn't very good as people were able to blow out the fuse by trying to use it to jump start a car with dead battery.
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Old 02-18-2013, 05:33 AM   #6 (permalink)
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Is your current limiter passive (resistor), linear (transiter CE junction that climbs in resistance with current) or switching mode (duty cycle/frequency which varies with current)?
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Old 02-18-2013, 08:24 PM   #7 (permalink)
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Quote:
Originally Posted by addertooth View Post
Is your current limiter passive (resistor), linear (transiter CE junction that climbs in resistance with current) or switching mode (duty cycle/frequency which varies with current)?
The design I am using now is a MOSFET driven by an op-amp doing high-side current sensing. I had all the parts in my lab so it was an obvious choice for me. The only thing I don't like about the design is the current-limit needs to be 'calibrated' because of the device-to-device differences in MOSFET characteristics. The other drawback, in terms of charging time, is the charge voltage is always less than the source battery voltage. However this design is inherently safe for the target battery.

If I was doing this type of product in higher volume then I would probably go with a dedicated switcher-mode power supply. There are some pretty nice single chip IC boost converters that can do 5 amps. There are also dedicated ICs found in commodity lead-acid battery chargers that could be connected after the boost switcher to provide the reverse polarity, under, and over voltage protection. Lots of options. Configure the boost for ~18V and then let the battery charger IC do this rest.
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Old 02-19-2013, 12:02 AM   #8 (permalink)
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It makes as much sense or maybe more sense to put an Anderson plug on the rear clam and also on the DD and then you have a better connection and less stuff to go wrong.
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Old 02-19-2013, 01:31 AM   #9 (permalink)
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I regularly trickle charge through my cig lighter with a some Mercedes branded tender/wall wart. If you wanted to go ghetto, you could plug a cheap inverter into the source vehicle, then chain the tender to it. Obviously not the most efficient method though.
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