Plugin LEAD acid owners

Look up Cheap! using the search function. He has lots of info.

 

Lead Acid is not worth it. You need to go with Hymotion or a DIY Lithium conversion. I have now converted more Ford Escape Hybrids into Plug-ins than Prius, but I think for some, a Plug-in Hybird is worth the effort. I know it is for me.

 

It does seem that the current PbA designs are severely hampered by battery life issues. I am not convinced that this is an inherent problem though. It seems like a result of over-discharge, and lack of balancing. From the EV world, the general rule of thumbs I've gleaned is never take a battery below 10.5V, and if you use sealed batteries you need balancing at least on charge. The current cal-cars design ignores both of these points, in an effort to keep costs down. I've heard discussion of the batteries getting down in to the 8.5V range during charge sustain mode, which is well below 100% DOD/0% SOC. Thats certainly going to have a negative impact on battery life. It also seems like when failures occur, its one or two batteries that fail first first, not the whole pack. I believe this is a classic symptom of system imbalance. As the SOC level of the batteries drifts apart over time, some batteries will be discharged more deeply than others. This is particularly bad news in a pack that is being over-discharged. The weekest batteries will start being pulled so low that their weekest cells are reversed which rapidly accelerates the demise of the battery. Worst of all, once a few batteries go you pretty much have to trash the whole pack. The batteries need to age all together in order to maintain any sort of balance, so you can't generally just replace a few and go on.

I like the newer battery technologies in general, but I think that there is a real need for a robust inexpensive conversion option. It should be possible to get a lead acid solution down into the $2-3k range for PHEV10-12. I think it will be quite a while before NimH or Li-ion can match that, at least in the conversion market. If the lifetime issues with lead acid can be solved, I think this would be a good bottom end option.

When if ever I get the time/money to do my conversion (between two small kids and the economy it may be a while). Here is what I have been thinking about:

Batter Pack: $1000
Standard Cal-Cars recommended SLAs, ~20Ah 12Vx20

Charger: $500-$700
20 individual isolated 12V SLA smart chargers. This ends up being cheaper than a bulk charger, and provides balancing by ensuring that every battery gets the proper amount of current

HV and LV electronics & wiring $500-$700
Standard cal-cars contactors and SOC spoofing controller. Need to add cheap uC to monitor voltage of each battery and kick the contactors out when any 1 battery drops below Vmin. Also should add a simple monitor to each charger, to lock out PHEV operation if all chargers did not complete cycle. This could also be implemented using the same uC monitoring battery voltage too. Every battery would have to get above a certain voltage to be considered "charged".

Battery box & mechanicals $500:
Either standard cal-cars, or I've been drawing a concept for battery bricks of 4-6 batteries that stack together, but can be disconnected and removed to access the spare or for long trips.

Total ~$2500-$2900. I'm hoping that with the changes made I can get 1-2 years charging twice a day on a 9 mile commute. With a replacement battery cost of ~$1000 that would be ~7.5-15c per mile which is approaching reasonable.

Of course, by the time I get around to doing anything, NimH or Li-ion may be affordable.

Rob

 

Rob - I think you're spot on.

Also, to help with the cell imbalance issue when the cells start getting low, there needs to be a way to bypass the underperforming cells to 1. avoid overdischarging them and 2. maintain performance.

By bypassing the dead cells, you can maximum output from the rest of the pack.

Interestingly, this sounds like the type of thing that National Semiconductor recently announced for solar cells called SolarMagic. The technology maximizes output from solar arrays when parts of the array are shaded. I assume that they do this by bypassing the shaded cells, if so, then this is very similar to what would need to be done for battery packs.