Has anyone tried running 2 smaller lithium batteries in parallel?
 

Simulating the dual Rebats behind the seat. Hearing about potential cell imbalance issues with that...

why in parallel?

you can replace the old acid with a light weight 12v lithium for cars - just dont let it run flat.

Depending on the plate size, one of the smaller batteries may not give enough cranking power if the engine has to turn over several times. Putting them in parallel will double that.

Ron

Lithiums
 

Right Ron, there appears to be a big price gap between small lithiums and one large enough for bigger engine needs. Ones similar in size to the Rebat are available for around $250 and then larger lithiums look to go for $1500 or more. This ends up being nice for a Cobra as it approximates the look of the Rebat installation with two. I'm actually looking at using these in a boat right now, but use in the Cobra is a concurrent possibility

That is a good way to find out how they work. If they work out well in your boat then you know they will work in the Cobra. Good luck with them.

Ron ;)

I'm running two 12v gell-cels behind pass seat as delivered by Kirkham in 1997. (although not the original batts) Have been working fine cranking a BB with geared starter.

Have you considered a battery isolator? "Single battery isolator utilizes the energy produced by the alternator and distributes the power evenly to each battery. Single battery isolator allows primary batteries to charge together but isolates each battery during operation." This will maintain your starter battery and allow only the starter to be driven from that battery even though your electrical system is run from perhaps a smaller Lithium battery. I would think there should be a device on the market for connecting batteries in parallel to control overcurrent when they equalize after loading or charging. I would never recommend different sized batteries connected together without some kind of regulator device between them.
Same Kind batteries in parallel should not be a problem as long as you do not have more than a few volts difference between them. Normally, your charging system will have 14 - 15 volts to charge your battery, while your battery will have 12-13 volts at rest. The 2 batteries will equalize over time, one discharging into the other. The main problem will be at starter demand, if the 2 batteries do not discharge at the same rate under load, then you can get current loops even after starting has completed. Potential differences in the cells can have high currents which result in heat and maybe fire. Exploding batteries is not good for your Cobra. I guess the major concern I would have is if you have 2 of the same batteries and one goes bad causing discharge into the bad battery. That would probably be the worst case scenario. And that all depends if your battery shorts...?
Isolators just create 2 or more circuits in your car. Battery 1 for starting, Battery 2 for other items.

Control software
 

Xack - I spoke with a tech person at the lithium battery manufacturer and he said two of the same models of their battery would have a lot of cell imbalance problems running in parallel, resulting in significantly reduced performance. He mentioned a BMS (Battery Management System) as being needed to optimize this. I'm just not convinced how big of an issue that is, for example he even said their batteries were not good for an application like a Cobra or boat and when I pressed him why not he just said "we haven't tested them for that". As opposed to initiating a whole research project, I just thought someone out there has probably been through all this. The lithiums sound like they could very well be a much better option than even batteries like gels and dry cell lead acids, in large part because of the weight savings... a typical group 45 battery like used in a Cobra weighs around 45lb and TWO of the Rebat size lithiums only weight a total of less than 10 lb.

DMXF,

I am going to assume that the chemistry that you are looking at using is LI-FePo4 as this is the normal for large scale cells and readily available.

The problem is cell balancing as you were told. The individual cells are 3.2 volts at full charge. They can be cycled down to 2.8 volts without damage. What you run into is during charge using normal 12 volt chargers (This assumes that you have 4 cells in series) is that it will not balance the cells. As you are charging the pack in reality one cell may be 3.0, another 2.9, another 2.6, and 2.8. They will come up in equal volt amounts.

In other words each one will gain the same amount of charge IE: .3V This will give you a cell at 3.3, (Definite No-No) one at 3.2, one at 2.9, and 3.1 giving a pack voltage of 12.5 which the charger will assume is full charge. but only one cell is at full charge. With one too high and the rest low.

As you cycle the battery this imbalance just gets worse until you fail a cell either from over charge or over depletion.

That is why you need a BMS for LI batteries. The system will individually charge each cell to the correct voltage.

There are suppliers out there that make 12V packs for automotive use that have internal BMS that will work for your application. Look under racing Li-FePo4 12V sport batteries.

I agree, you don't want a battery fire because your battery experiment went wrong. Just get yourself a red top and be done with it.

Fork lifts cells have lead-acid batteries in series and parallel. They probably have higher tolerances to be used in a group like that. You should pay attention to this issue of voltage imballance when using lithium ion batteries with these load ratings. Lithium ion batteries are known to cause fires. Just be careful.