redcelt007 wrote:According to the MotorCraft service manual, the battery is rated at 43 amp hours.... Using a lithium battery would be risky, in my opinion, because of the charge is limited to 80-90% of full capacity. That dc-dc converter might not charge lithium properly.
Thanks for the info. Is there an actual manual other than the web based stuff? I'd like to get my hands on one.
I will probably buy a deep cycle AGM as hybridbear pointed out hybrid/energi owners have done.
At least the A123 based "12V" automotive batteries would be fine with the DC-DC charging. These packs are made by putting four cells in series and then adding addition cells in parallel to get usable capacity. You will often see them denoted as 4s2p, the number before the "s" indicates how many cells are in series and the number before "p" indicates how many cells are in parallel. So in the example I gave (4s2p) it would have 4 groups of 2 parallel cells in series for a total of 8 cells. The following specs are from memory while researching the feasibility f using a lithium pack in my motorcycle so I may be slightly off. A 4cell A123 based pack is nominally 13.2V. Each cell can safely handle up to 4v charging so as long as the DC-DC converter doesn't put out 16V+ (max converter output ~14.5V) it would be ok. Since the cell voltage never reaches 4v the battery is never charged to the theoretical 100% This video of me starting my motorcycle in cold weather demonstrates why lithium starter batteries (at least for ICE vehicles) have not caught on.
Additional reasons not to use a pack like this is the potential for cells to not be balanced (all cells have same state of charge). It would take a complicated battery management system to keep things in balance. Also without battery protection circuitry, if you ever were to accidentally run the battery down, it would be destroyed and never hold a charge again. To build a 41.4Ah (each cell is 2.3Ah) pack for our car would be a 4s18p battery. That's 72cells! An 8 cell pack is about $100 so I imagine a 72 cell pack would be in the neighborhood of $900!!! Not a viable option!
I'm not nearly as knowledgeable about the larger higher capacity prismatic lithium cells, but I suppose it would be possible to make a 4 cell battery with them where you wouldn't really have to worry too much about the cells being balanced assuming they are balanced when the pack is assembled. From some quick googling for a 40Ah battery it would be 4x$120 cells = $480 just for the cells to build a "12v" battery! I don't know anyone who would spend that much instead of buying a cheap lead battery. It would cost even more to add protection from an accidental complete discharge. Additionally who knows if these cells can tolerate the 14.5V charging voltage from the DC-DC converter? It may be necessary to add a protection circuit for overcharging adding even more cost!
I just saw another site that listed the 40Ah prismatic cells at about $50 which makes a 12v lithium pack more reasonable.