# Should I check battery current by ammeter?

I had the concept that in order to check the maximum current a battery can supply, it is fine to connect an ammeter in series with battery because ammeter has low resistance in series and this will yield the maximum current a battery can supply.

Many people have said it is wrong, but I can't understand why.

If you put a voltmeter in parallel with the ammeter you'll see that the voltage is very depressed.

What you need to do in order to measure maximum current available is to measure it through a variable resistor while also measuring voltage across the battery. The resistor should be adjusted to the point where the voltage is at the desired minimum operating voltage and then the current read from the ammeter.

"Many people" are correct in this case.

The problem is that an ammeter has a very low internal resistance. (It is designed to measure the current with minimal loading effect on the load.) If you connect it across the terminals of a battery a large current will flow, limited only by the internal resistance of the battery and the meter - both of which will be low.

simulate this circuit – Schematic created using CircuitLab

Figure 1. The wrong way and the right way.

Instead, figure out what the battery should be able to supply, connect up a suitable load resistor or lamp which would draw that amount of current and measure the result.

Drawing short-circuit current from a battery can damage or even explode the battery even if it is done for just a few seconds. The only way to know the current that a battery can handle safely is to find the information in the manufacturer's specifications for the battery.

The reason for that is the internal battery parts have resistance. That resistance causes internal power loss in the battery that heats the components. The components can deform or melt if the temperature is too high. Designing those components to carry a certain maximum current is part of designing the battery.

Testing the voltage drop at various current levels can provide a good indication of safe operating current, but only the manufacturer can determine that with a reasonable amount of certainty.

• Understood. But theoretically, I can't see any flaw in checking current by ammeter(low ammeter resistance, maximum battery current). Is there any mistake in it? – Ahmer Nov 4 '17 at 14:57
• The mistake is that any ammeter will have a resistance that is too low it is essentially a short circuit. It will draw too much current for the battery to safely provide. – Charles Cowie Nov 4 '17 at 15:03
• @Ahmer: Read Charles' first sentence again. Does explosion risk not count as a flaw in your proposed technique? It can explode both in theory and practice. – Transistor Nov 4 '17 at 15:03
• Understood now. – Ahmer Nov 4 '17 at 15:08
• @Ahmer - even if the battery does not have a problem there is a high probability of damaging your meter, on the lower current ranges there is usually a fuse. You should not get in the habit of using the current ranges of the meter except in circumstances where you know what to expect. – Kevin White Nov 4 '17 at 16:38

When you connect an ammeter into an automotive circuit there will be a current draw well in excess of 50amps which will blow up multimeters. This is due to all of the modules on the vehicle starting up!

• Shouldn't, it's over voltage that kills things, most meters are rated for hundreds of volts. If measuring current (while the meter is in series) over current condition would cause the fuse in the meter to fail, and those are replaceable – Voltage Spike Jul 3 at 20:10
• @VoltageSpike He's essentially correct. Both the OP and this answer are talking about current drawn from the battery. To measure this the OP is talking about placing a DMM on its high current range in series with the battery lead. Most DMMs withoutr a special shunt are rated at 10A or 20A in some cases. These are almost invariably NOT fused. If current usefully exceeds the meter rating damage or destruction may occur. This answer suggest 50A - power = 50A x 12 V 600W. which is high if not starting. Typical may be 100W headlights, say 20W other lights, audio amp MAY be high, ... – Russell McMahon Jul 4 at 12:35
• ... indicators, fans, ... . Say 200W - 16A would be highish . A 0A meter (most) would be well over range and starting to get warm quickly. Hit the starter and the meter would die. || WORSE: THe OP is asking about battery current capacity testing by placing the meter across the battery. He does not mention vehicles For an AA or C cell this is doable. For almost any form of lead acid and most Lithium batteries it would destroy the meter. – Russell McMahon Jul 4 at 12:36
• @russell, don't meters have fuses to prevent over current from damaging the meter? – Voltage Spike Jul 4 at 16:44
• @VoltageSpike I decided to add an answer. In eg Fluke brand meters there probably is a fuse or electronic protection. (I've never dismantled a Fluke meter to check). For most DMMs under say $US100 there is almost always either no fuse or at best an informal fuse - see photos in my answer. – Russell McMahon Jul 5 at 11:20 Testing a battery's current supply capability by shorting it with an ammeter is a very bad idea in many cases, and an effective but informal method in selected cases. Where it works: For Alkaline and carbon zinc batteries in the AA size, short circuit current capability is usually inder 10 amps even when new. A short duration short circuit with a multimeter on its 10A or 20A range will give a good indication of the battery's state of charge. The short circuit should be for less than 1 second. I have used this method for many years for testing only this class of batteries. It provides a rapid and effective means of assessment of battery state. For AA NimH cells this will usually produce less than 10A but the energy available from the cells makes this a marginal act. Where it is a very bad idea: For most other batteries - larger Alkaline etc (C, D) or LiIon, lead, acid and other higher energy cells applying a short circuit will be liable to damage or destroy the meter and may damage the battery - possibly dangerously. High quality meters costing typically 100's of dollars US (or equivalent) may have over current protection on their high current range. This may be via a formal fuse or perhaps will utilise electronic circuitry to prevent damage. Most low or medium cost multimeters do not have formal protection against overcurrent on their high current range - usually rated at 10A or sometimes 20A max. significant overcurrent will usually damage or destroy the meter. The 10A arrangements for two meters are shown below. The first meter has no over-current protection and is typical os most meters that cost say under$US100 - and possibly rather more. The second meter has a wire link acting as an informal fuse.
This is a higher rated and priced meter than most non-main-brand meters.

Typical 10A current shunt in low to medium cost DMM.
There is no fusing and excessive current will probably fuse the track on the pCB between the shunt and meter jack.

10A current shunt and informal fuse on a somewhat more upmarket meter.
Ends of 10A shunt ringed in green.
"Fuse" consisting of wire strands circled in red.