Vehicle electrical Q (or how do I use a multimeter)

We have a 1995 Mazda MPV Beater Van. The battery has gone completely dead (i.e., no lights, no horn, nada). It’s been to the mechanic once, battery is new and alternator checked out fine—rock on little van. But the battery died again (it had sat for about a week and a half).

Disconnected the battery, put it on a charger, let it charge for several hours, and still had no electricity. Suspicious (details to follow), I simply jump-started the van and it roared to life (rock on little van!). I let it run for about a half hour, shut it off, it started fine. Took it out a few places over the past couple days, no problem starting.

Suspicious thing about the charger was that it’s built-in ammeter showed that the battery was full and that it wasn’t drawing any (current? amps?) from the charger. The meter’s needle is supposed to move from right to left, with 0% charge and 15 amps on the right, 100% and 0 amps on the left. But it was stuck on the left all the time. Can I use a multimeter to check out whether or not the battery charger is kaput?

Suspicious thing about the van is that something is eating the battery. It could be as simple as an ajar door leaving an interior light on (the van only gets driven once every two weeks or so), or it could be some larger issue with the electrical system. Can I use the multimeter to find out if there is some draw on the battery with the van off and after being sure all doors are shut?

Oh, maybe it would be prudent to state that I have no idea what a multimeter is, what it’s normally used for, which end of it is up, or if it’s the right gizmo for what I’m trying to figure out. I just picked one up from Lowes, but it’s unopened, so I can return it in case it’s unnecessary or I need some other gizmotronic device. If this was the right thing to get, can someone kindly tell me in nice, simple, idiot-proof terms what I should do with it? (Man I hope multimeter settings are fairly universal!)



If you believe something is “eating away” at the battery when the vehicle is off, you will want to verify this by measuring the current from the battery when the vehicle is off. Your digital multimeter (DMM) should be able to do this.

Shut off the vehicle and then disconnect the negative battery cable from the battery. Configure your DMM to measure DC current. If you’re uncertain on how to do this, consult the manual for the DMM. It may require you to plug the leads into specific jacks on the DVM (different from the jacks for measuring voltage). If you’re still confused, provide us with the make and model of the DMM (or something close to it). At the very least tell us where you purchased it.

Rotate the knob on the DMM so that it reads the highest DC current range. Connect the DMM’s black lead to the negative post on the battery. Connect the DMM’s red lead to the black battery cable. Read the current. If it’s reading zero amps, rotate the knob to a lower DC current scale until you get a reading. If you get 0.00 mA when the knob is rotated to the lowest scale, then the problem might be something else.

The multimeter could be helpful in measuring the battery’s rest voltage. Other than that I doubt it will be much help.

If the battery was totally dead, that could keep the charger from initiating charge on it. My experience is that many chargers won’t start charging unless there is some minimum level of residual charge in the battery. With chargers that have a “start” (as in jump start) feature, using that for ten seconds or so is usually enough to kick start the process. Alternatively, you might be able to get it going by hooking the dead battery in parallel with a good one. Let it charge for a few minutes, then disconnect the good battery and see if the charger is charging the dead one. Anyway, don’t condemn the charger until you’ve tried it on a battery that is not dead.

Likely there is a significant drain on the battery. An DC ammeter with sufficient range (at least 10 amps) could be very helpful in tracing that. However, my experience is that such ammeters sell for several hundred, and I seriously doubt that the DMM you’ve bought has the range. It is possible that your drain is 2 amps or less, and it is possible that your multimeter will measure that range, but you’ve got no way of knowing how much drain you’ve got before hooking up the multimeter, and if it exceeds the amp range on the instrument it may burn something out on it.

If you do have an ammeter capable of reading the drain, then that allows you to go through the process of disconnecting various circuits, and then maybe various components, one at a time to see where the drain lies. Doing this effectively requires some basic understanding of what’s going on and at the least an accurate and complete listing of what circuits are served by which fuses. A full wiring diagram is sometimes needed. If you really think you’re up to this, how to use the multimeter can be easily enough described.

you can also do similar to measure the current going from the charger to the battery.

when using the meter to measure current there might be a specific jack for measuring current as mentioned, there also might be one for higher amounts of current for this purpose (it might be labeled something like 10A).

a make do solution could be to disconnect the battery when it is not in use, until you solve the problem. reconnecting a battery cable is as much work as jumping and it is better for the battery.

I have an 1984 4X4 that sits for months. It kills batteries. I have side mount connectors and I keep a 1/4 ratchet with the correct socket laying on the battery and the hood cracked.

Pop the hood, connect the battery and go. Less than 60 seconds. Through for that week / month. pop the hood disconnect the negative terminal and lower the hood with the ratchet laying on top the battery.

Bad part = none. Just an analog clock that I do not want to unhook. I could put a switch in that line but…

Also, any battery that is not used for weeks or months at a time will do better in the long run if left with the neg treminal disconnected …