Foo - Mixing batteries with different MAH bad?

I have four batteries, two 1500 MAH ones and two 1800 MAH batteries. The one 1800 is a dud and hardly holds a charge anymore and the plug on one of my 1500's is bad. If I were to run a 1800 along with a 1500 in my RC Truck would I damage it? I'm on a tight budget at the moment and cant afford another 1800.

I think.... that if you do that, it won't damage the RC truck, you'll just cause a larger strain on one of the batteries than the other and eventually, after repeated uses and charges, they will probably not last as long as if you used to like MAH batts. I'm going off of my experiences with our Motorola batteries for our radios at work, they all have different MAH ratings on them, but of course the voltage is all the same, but the ones with the lower MAH levels sometimes will not last an entire eight hour shift.

Repair one of the 1500ma packs by using the connector from the dead 1800ma pack. Now you'll have three usable battery-packs.

Running two packs in parallel will be fine and will almost double your run-time.

The thing to be careful about is the individual cells within a battery pack. Those MUST be of exactly the same capacity AND charge-state at the time you assemble the pack. Or else one of the cells will be fully discharged before the others and end up getting reverse-charged.

Fix the bum plug. If you have a good cycling charger, see if it can resurrect the bum pack.

If your truck takes two packs in parallel, any difference in voltage level can be harsh on the packs. If the voltage isn't equal, the parallel connection effectively causes the higher V pack to charge the low V pack at heavy rates. NiCads can probably handle this, NiMH maybe. Lithium polymer packs can go boom. They usually have special circuit devices to allow safe parallel connections.