Hi all,
I'm fairly new here as I've recently bought my first C5!
It has an original style battery with the original keyswitch etc, but I'm planning to replace this.
These are the cells I'm considering, 3.2V, 60Ah:
http://www.sinopolybattery.com/ClientResources/201207251418462.pdfI plan to stick four together by their largest surface (not sure how? Maybe just duct tape? suggestions welcome) and connect them in series.
This would give me a pack with dimensions 244 x 116 x 185
Dimensions of an 038 battery which I found on the internet: 238 x 135 x 202 so my replacement battery pack would almost fit entirely within the dimensions of the original! Perhaps I could keep the original keyswitch if I wanted to...
In fact, I thought I'd make up two such packs, one for each footwell, and connect them in parallel to give 120Ah. Compare this with the 15-20Ah (after peukert) of the original battery.... I could have around eight times the range.
Voltage is 12.8v nominal, with 15.2v fully charged and 11.2v once flat. Internal resistance would be 1.6mohm
Since these are LiFePO4 rather than Li-ion, they're a lot more resistant to overcharging and overdischarging - possibly more so than a lead acid battery.
So to sum up the advantages:
- Greater range - possibly eight times
- Slightly higher voltage - bit of extra speed
- Lower internal resistance - I tend to find my C5 gradually slows as it's going along, then if you lay off the throttle a bit and accelerate again, it gets faster. I've attributed this to accumulation of PbSO4 near the plates giving a higher internal resistance, which gradually dissipates when you aren't drawing current. Hopefully the lower internal resistance of the lithium cells will avoid this frustrating decay of speed
- Lighter - well, not exactly. Each pack individually will be 8kg, but the two together will weigh more than the 11-12kg of the original battery
- Maintenance free - no acid to top up (although this is also the case with VRLA...)
Now, here are some problems I figured I might encounter:
The fully charged open circuit voltage is higher. 15.2V is just outside of normal lead acid range so I wonder if this would damage the electronics, eg in the pod? The ULA, as I understand it, runs off a 7805 linear regulator so should be fine though (can anyone confirm this?)
I believe something in the pod electronics is callibrated for the internal resistance of the battery - possibly the fuel gague? Anyone have more info on this? Can I recallibrate it just by changing a resistor value?
The lithium iron discharge curve is very flat but also very nonlinear - I wonder how the fuel gague would deal with this?
In light of the above, I may have to completely rebuild the pod electronics using a more modern microcontroller. I'd like to keep it outwardly authentic though, so it still behaves in the same way. But a more accurate measurement of battery charge (as could be acheived with a microcontroller ADC and a lookup table) would be quite helpful.
I haven't really considered how I'm going to charge it yet but I think that should be a non-issue: lithium iron chargers seem to be fairly easy to come by.
So basically I've posted this just to ask - what do you guys think? Can you forsee any other problems? Is there anything I've neglected to consider?