Charging off car battery while car is running? - RC Groups
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Sep 19, 2003, 08:09 PM
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Ben74's Avatar

Charging off car battery while car is running?

hi all. after killing my 115Ah deep cycle battery in less than a year, i am trying to think of ways to get large scale charging capacity at the field. when the deep cycle was brand new, i could get about 8 charges on my 24 cell heli packs. however, i expect to be charging even bigger packs soon, and would like to be able to spend all day at the field without running out of juice.

i'm wondering if there are any major issues with charging off my car battery while the car is running. is the power going to be "clean" enough? if not, would using an inverter and an AC to DC power supply help smooth out the power? my vw passat is quiet and non-stinky, so i figure i could leave it running without bothering anyone.

also, how many watts would it be safe to pull from the car while it's running? i'm going to need about 400W max. i am currently using an astro 112D (~240W max), but i'd like to run two astro 109 lithium chargers in the future (~180W each).

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Sep 19, 2003, 08:43 PM
Registered User
I would be concerned about the alternator being able to "keep up" with the charger @ 400 watts,especially with the car idling rather than running a higher rpm.You would probably be discharging your car battery anyway,and maybe harming your alternator,by asking more of it than it is designed for.I have read on here previously from automotive technicians,that alternators are designed as "maintenance chargers",and not designed to recharge a partially discharged battery very frequently.Maybe someone will chime in with some more knowledgeable info.If you get another battery,check out the Optima line.I think they even offer a parallel battery setup.That ought to be fun to lug around.
Sep 19, 2003, 09:17 PM
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Ben74's Avatar
you know, if you think about it, the alternator had better be capable of doing at least 400W. the two amps in my stereo system are rated at 200W and 220W RMS, the headlights must be sucking at least 200W each, then you have the car's computer and everything. if the alternator isn't capable of 400W, then my car wouldn't run at night with the stereo cranked up (do it all the time).
Sep 19, 2003, 09:39 PM
Registered User

i charge my nimh when my car is running and no problems yet.
i just got a shulze 330 today and the instructions say do not have the engine running while you are charging.
you could start your car periodically to refresh your battery.

i also keep a jump box with me just in case i kill my car battery.
see this thread....

i also use the jump box for tx and rx charging too.

Sep 19, 2003, 10:24 PM
Registered User
Look around for a Ham they run a few watts with laarger batteys
I had a xmitter I ran 24v sys i would pull 110 amps at 24 v had 2 big D12 batterys 400 amps now that was deep cycle. then the filment/ heaters for the was 10v at 50 amps Hams do use some power there are chargers that can be set real close for poly mine is home built at 12 v i can charge at 20 amp or inbetween
but also i charge to 3.18v x 6 cells Maybe a local ham can help you. CU
Sep 19, 2003, 10:35 PM
4 Point Landing
fred420's Avatar
I run 3 ST's at once off of my truck battery. I can hook 3, 9 x 2400 NiCd's and charge them at 3.5 amps. I have ran my ST while having my truck running and the only difference that I see is in the input voltage which inturn relates to shorter charge times. As an old ROAR member in the 80's, I was a sponcered driver, we all ran our vehicles while charging.

Just my thoughts,

Sep 19, 2003, 10:46 PM
Registered User
I usually charge on the way to the field with the car running.My Orbit V6 and Orbit Pro will shut down with over voltage (>15v).They only do this when the car is first started,after a few minutes they will continue to charge.I am only charging 10 cell packs and smaller.When I get to the field I use an Optima battery to charge.

then my car wouldn't run at night with the stereo cranked up (do it all the time)
Do you ever leave the car idling for a couple of hours with the stereo cranked.If so, with no problems,maybe the charging won't be a problem either.Try it and let us know how it works.
Sep 19, 2003, 11:38 PM
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Ben74's Avatar
won't the alternator increase it's output and increase the motor rpm to match it when the electrical load increases? if i turn on my headlights and fog lights while the engine is idling, i can hear a slight change in motor rpm.

i will call bob at astroflight on monday and get an answer from someone who should know.
Sep 20, 2003, 12:37 AM
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ScotY's Avatar
Hey Ben,

I asked this question recently in one of the other threads. Ask Ulf and he will tell you. I am about 75% certain he once told me it was okay, provided you started the car FIRST, then connected up the charger. Come to think of it, it might have been someone else who told me that but I'd ask him anyway.

I would think almost any healthy alternator should be able to generate 20+ amps even at idle. This is just a guess but I doubt it's far off. A slight increase in rpm is not likely to increase this, I don't think.

Remember, music is dynamic and so is the current your stereo is drawing. Eventually, someone is going to go and get a high output alternator installed in their car to charge their lipos at the field. Any decent car stereo shop can help with that. They require clean power also so I would think it would be okay to do. I installed a 100 amp alternator in my car a few years had a speed up pulley so it would put out more at idle...too old for loud stereos now.
Sep 20, 2003, 12:43 AM
Registered User
aau007's Avatar
Car alternater cranks out 13.8v. You are drawing roughly 30A at 400W. You should check with VW dealer to see what's the output on the Passat's alternater.

After confirming with VW that your Passat's alternater can sustain the amperage, then proceed to next step.

Next Step: You DO NOT want to use the cigarette lighter plug as most of them have only a 10A fuse. Using a little clamp on the battery terminal can be tricky for 2 chargers, each drawing 15A. When you clamp onto the terminals using those clips from the charger, you usually get a 4 points contact and passing 15A through them may be finicky. Best if you can splice into the fuse box with a 20A fuse for each charger.

Trying to use unleaded gas to power the charger may prove to be too expensive in the long run though. Besides, idling your car for extended period of time is not good for the engine.
Sep 20, 2003, 12:47 AM
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Ben74's Avatar
the voltage at the battery terminals while the motor is running is about 14.1, measured with a DVM. my astro charger says it takes 12-15V input. no problems there.

why would idling the engine for an extended period of time harm the engine?
Sep 20, 2003, 12:54 AM
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ScotY's Avatar
It's about the same as sitting in LA traffic! Better to go to an alternator shop...they can put a load on your alternator at idle and tell you how much it can put out.
Sep 20, 2003, 03:36 PM
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aau007's Avatar

It is not the voltage. Car alternators are regulated to output between 13.5v to 14.5v in general. It is about the current draw it can sustain before you blow it.
Idling the engine too long causes more wear on the cyclinders (than cruising 70mph at top gear) and in the long run, it means that the engine will lose power.


While testing the alternator at a shop can tell you what's left on its life, simply asking the VW dealer for information costs nothing. Besides, do you want to stress the alternator without knowing what the spec is supposed to be?
Sep 20, 2003, 04:20 PM
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Arthur Knowles's Avatar
As an FYI, the main problem with charging while the car is running is voltage surges. Many, especially computer driven chargers, do not like voltage surges. If a high one occurs while charging it can blow your charger.
Sep 20, 2003, 08:45 PM
Registered User
I did a quick Google search and found a page on testing the alt. on a Passat.When testing you should have 70 amps at idle(1000rpm) and 90 above idle.

Someone could look here and see if I am reading this right.