need help on battery power

[Gallienus]

I have an RC car for my young son which uses a rechargeable lead-acid battery. Battery is 6V, 4.5 Ah/(20Hr) by First Power. This battery powers his car & him until it runs out (complete discharge). Battery size is like a deck of cards only 2X as thick.

No clue what 20 Hr means, battery generally lasts about 30 minutes on a full charge before dying completely. After very few uses I think the battery useful life is decreasing.

I wish to replace/upgrade the power system. I have in my closet 4 Metal Hydride rechargeable D-cells (1.2V 10 Ah) + 4 NiCd D-cells (presumably 1.2V). The car also uses a standard charger which plugs into the car. Taking the battery pack out is too complex for everyday use: I'd like to use the plug & the adaptor, which is probably not a smart charger (I do have one but this requires putting the batteries in it). Thus I can recharge the car without taking the battery pack out.

I have 2 choices: replace the lead acid battery with D-cell or other rechargeables or buy another (much larger) 6-V lead acid battery.

Any recommendations? I've heard that lead acid batteries are killed by deep discharge which is exactly what this car gets. Thus I'm not inclined towards another lead acid. On the other hand metal hydride batteries like deep discharge but I'd need to put 5 or 10 of them together and use a standard 6V charger. Also I've noticed on-line that they sell 7.2V 4.2AH battery packs with a smart charger. It says not to charge the packs together, however, there really isn't any other way. Also if I put 2 packs together I can have 8.4 Ah. The motor may run a bit faster but hopefully things won't burn out at 7.2 V instead of 6.0???

The local RC store (Warricks hobby superstore) blew me off and said "junk like that is not interchangeable: buy a whole new car at toys-R-us, we don't deal with such things: Goodby"

Anyhow any insights would be appreciated. I have a [chemical] engineering degree so I should be able to rewire stuff if needed.

Thanks!!

[ELECTRONTRAV]

6V 4.5 (20Hr) means you will be able to get 4.5 ampere hours out of the battery if it takes 20 hours to discharge it to the normal 1.8v per cell,
in your case 5.4v, if you go below this, into deep discharge it is a battery killer. The higher the current drawn from the battery the less efficient it is
so you are probably drawing about 8 or more amps from this battery which is 0.5hr rate which is no gopod for any lead acid battery. if you can get a
relay in circuit to cut out at about 5v your battery will live longer, but more capacity and a relay is the better answer. I am not sure deep discharge
on NiMH is a good idea either. Also a fully charged 6V lead acid battery can be about 6.7v

[Gallienus]

After some consideration I decided to replace with another 6V Pb-H2SO4 battery rather than a NiCd pack. The new battery is rated at 12 Ah instead of 4.5. Unfortunately after installing it the car does not work: the only difference I can see is that the old battery was soldered into place & I just reattached the clips rather than solder them. I'll check it out w/ a volt meter to see where the circuit's bad.