Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Charge battery while still in car?

If I need to charge the battery in my truck (with my battery charger), can I do that with the positive and negative terminals still connected to the battery??


In fact, I recommend it, because you won’t lose your radio and clock settings


I always do.

1 Like

Is this the vehicle that you had trouble with before and installed a new battery? If the alternator is good and you are still making short trips you may need a trickle charger.

I hadn’t thought of that. Say, many thanks.

Thank you, Bing. (One of my favorite names; I’m a real fan of Crosby.)

Boy, you have a good memory. :slight_smile: Yes, it is. It’s now got a new alternator in addition to the new battery. I guess I have a “parasitic [ amperage ] draw” somewhere. I just bought a pretty good multi-meter. I’m going to pull the fuses one by one till I find out what circuit and/or accessory is the culprit. Then go from there.

I don’t know if I have a trickle charger or not, but I did buy a battery charger; I’ll have to read the manual and see if it will do a trickle charge. What is the advantage of that?

And thank you for replying!

1 Like

How large is that parasitic draw?

I haven’t measured it yet (and that’s why I bought the multi-meter). I’m just giving this my best guess, but I’d say a fully-charged battery lasts … three days?, maybe 15 or 20 starts before the battery has to be re-charged. I actually have two batteries, and depending on how critical my need is to get somewhere, I sometimes switch them.

[ Just FYI: Because this town is so small, about the longest round-trip you can here - say to the library and back - is only 6 miles. I do occasionally drive to the nearest gas station, and that is 36 miles round trip. ]

I have cars that sit for a few months, and they start up just fine without being charged first. With that in mind, I would think your parasitic draw is significant, perhaps several hundred milliamps

The fact that you apparently only do very short trips, also isn’t helping to keep your battery fully charged

I bought this particular multi-meter because it has a 200 mili-amp scale (per a really good car guy on YouTube). There are real towns both east and west of here, 50 miles in either direction. But I hesitate to make either of those trips because I don’t trust the batteries to get me back home, and I can not afford a tow to get the truck back home. … [ Probably more detail than you want. :slight_smile: ]

I also figured that the short 6-mile round trips around here were not doing the battery any good.

I have to get up at 4am; I’ve a “job” on the internet, so hopefully we’ll connect tomorrow. Thank you for your help!

I like using the test light myself and as you pull fuses see which fuse causes the light to go out. Some of the usual suspect though are trunk and glove box lights, etc staying on. Also I had a problem with the door handle shorting out causing the interior lights to go on during the night. (The kind where if you pull on the door handle, the interior lights go on.) Also had a bad electronic level control that shorted out. It was intermittent too so just found that by chance. First thing if you can isolate the circuit anyway.

You have to be very careful when measuring the parasitic current with an ordinary multimeter.

When the engine is running, the current is many amps, 10 amps at a guess. Starting, it is about 100 amps. After you turn the engine off, the current can still be high, perhaps several hundred mA for a while until all the electronics times out and goes into sleep mode, which may take hours. At that point, and only at that point can you measure parasitic current.

And, if you interrupt things by changing scales or connections, it may “wake up” the electronics and you have to wait again for them to go to the sleep mode.

If your multimeter is like mine, 200 mA is the highest range. There probably is a 10 A range, but that requires using a different connection on the meter.

How to do this: starting with the engine off, disconnect one of the battery leads and connect the meter in series, on the 10 Amp range. Take a reading. If it is less than 200 mA, you can change connections and change range to the 200 mA range and wait for the sleep mode. If it is over 200 mA, leave it on the 10 Amp range and wait for the sleep mode. You won’t have a lot of resolution, but you should be able to read 50 mA with some accuracy (resolution is 10mA)

Edit: and use clip leads to attach the meter, not the pointed probes, as holding the probes for an hour without losing connection is not doable.

And do NOT try to start the car with the meter connected, you will burn out the meter.

Here is a nice guide

An idea I had not thought of. But I’m also curious about how much amp. draw there is. And I’ve checked all the usual suspects, many of which you’ve listed. And I’m praying this isn’t an intermittent problem. :slight_smile: Thanks for your help, Bing!

Just prior to my testing I’ll re-read this. Seems you have a handle on this, and I want to take advantage of you. :slight_smile: I made a point of getting a multi-meter that measures up to 20 amps, and one with clip leads as well as the pointed probes. I know I have to be very careful with all this so as not to screw up my new multi-meter … and in my old age, I’m very risk adverse, anyway. Many good suggestions, BillRussell. Thank you, sir!

I top off the battery on my Corolla w without removing the connectors all the time. I always first connect to the battery posts, then plug the charger in. When disconnecting I unplug the charger, then remove the clamps to the battery posts. To avoid sparks and electrical transients damaging the car’s electronics.

on the other hand, multimeters are a $10-20 item today…

Hooking up the terminals first, then plugging the charger last … that’s what I do (even when charging the battery out of the car … which I don’t feel I need to do any longer). Thanks for the reply, GeorgeSanJose!

1 Like

I disconnect my battery when I park it for the night. Because I’ll go months between bouts of driving I sometimes have a hard time starting; an outboard battery booster works well in that case.