NiMH Starting Battery?
 

There have been questions here regarding the use of smaller, lighter starting batteries. And I am even using such batteries. But I was thinking the other day: why not take it further? Why not forgo heavy, bulky, toxic lead-acid batteries altogether for NiMH batteries? I figure that if you used TRUE D-batteries (not the more common AA batteries in a D case), you could have a starting battery that is smaller and lighter than a small lead-acid battery, yet which has somewhat more capacity. Has anyone tried it?

You may find that the cold temperature output to be poor compared to Lead and the charging of a NiMh battery with multiple cells will also be a bit more complicated since you would be operating at 1.3 volts per cell you would need an 11 cell battery to prevent over charging them and some sort of balancing circuit on the cells to keep them all equally charged. There are "F" cells that put out 50 amps at about 1.1 volts that I have several of and I have to constantly charge them and balance them to keep them operational. They also self discharge in a few days quite a bit then stay partially charged after that for a long time. They are also not as efficient as a lead battery for energy in vs energy out.

What amperage do NiMh batteries supply? Also, what internal resistance difference is there compared to a Lead battery? I'm thinking that the charging system could have problems since a lot of car charging systems are no longer adjustable, and are preset for 14.4 volts. I believe that lead acids are 2.1 volts per cell X 6 cells, so they have a constant difference of ~1.6 volts charging all the time (14.4-12.8). With an 11 cell NiMh you'd have a very small trickle charge of .1 volts which is good and bad. Good for highway use, bad for frequent starts. Unless it's able to charge quickly, then its good all around I guess.

Starters can have a peek draw of 100 amps or more, if only for a few seconds, lead acid batteries hold up to abuse pretty well if given in short bursts, and if they are over charged they just boil and need more water added to them more often, unlike other batteries that burst, or burn up.
I suspect that the Lithium car batteries are something like the A123 batteries, a good solid cell that can handle over 120 amps for 10 seconds or more, and over 70 amps for around two minutes, but they don't handle being over charged well at all, so they need a battery controle modual that can handle the high dischargs.

NiMh cells can put out a lot of amps if they are big enough and the charging voltage from the alternator is not a problem if you use at least 11 cells in series to prevent the cells from reaching full charge voltage. They will not get a full charge at 14.4 volts so they will not get hot or go negative DeltaV - NiMh and NiCad cells drop in voltage as they reach full charge. They also do not suffer damage from being left partially charged like Lead batteries do.

Well sort of, there is the "memory effect" thing, but you can usually cure that by deep cycling them.

No you are thinking of NiCad cells - NiMh don't have a memory effect and you don't want to deep cycle NiMh to less than 0.9 volts per cell or they become damaged.

Usually my NiMH batteries that fall below .9-.7v never hold a charge again.

NiMH batts do have a memory effect but it's not nearly as bad as NiCd batts.

NiCd batteries are WAY better than NiMH when it comes to high current draws(anything over 1.5-2 amps per cell).

Only lithium ion and lead acid don't suffer from a memory effect and only lithium ion can be completely discharged and not suffer from it.

What about a small lawn and garden battery? You typically need 1 CCA per cubic inch plus 50 for the electronics. This battery is 10.6 pounds and 205 CCA. That should technically be enough to start a 2.5L engine.