I want to measure the current that can be supplied from a lead acid battery I have. (To know it's remaining current supplying capacity). To measure current I need to connect a load and an ammeter (multimeter, with knob to direct current) in series with the battery. Can I use another multimeter (I have two of them) with a larger internal resistance (knob towards dc voltage), as the series load, if I don't have any spare loads with me?
Figure 1. Test circuit.
- Voltmeters are typically designed to have very high impedance (resistance) and 1 MΩ and 10 MΩ is typical. This is so they draw as little current as possible from the circuit under test. As a result a voltmeter would be useless for loading a battery.
- Ammeters are typically designed to have very low resistance so they don't cause any loss of voltage in the circuit. As a result if you were to connect one in place of VM1 a very high current would flow and probably destroy the meter or blow the fuse - if it has one (and it should).
The correct way is to use a load and monitor the current and voltage simultaneously. This way you can calculate the power at any time and you can stop the test before you over-discharge the battery. If yours is a 12 V lead-acid battery then car bulbs are a handy test load and you can get 6 W tail lamps, 21 W brake lamps and 55 W headlamps and mix and match. These are readily available but have the disadvantage, like all incandescent bulbs, that their resistance increases with temperature so as the battery voltage falls so will the lamp resistance.
Yes I suppose you can but chances are quite good that you will ruin your multimeter. These usually have a 10A maximum shunt in them for measuring current.
The usual process for measuring large currents, like from your lead-acid battery are:
- Use a high-current shunt in series with the load and you measure the voltage across this shunt.
- Use a "clamp" style ammeter.