Topic: Lithium (LiFePO4) mod - advice wanted!

[alecjw]

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.pdf

I 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?

[dansretropod]

LifePo4 batteries are great. I run my C5 with a 60Ah 12v one made by ping battery and get around 45-50 miles out of it for just 7.5kg of weight (I have it mounted in the boot ).

The normal C5 control box does indeed have a 7805 regulator to drop the voltage down.  I'm running from an arduino control box so couldn't say how the slightly higher voltage will affect the original pod.  Maybe just replace the heatsink in the control box with a bigger one?  I think you'll have problems with the pod fuel gauge showing full all the time until a few minutes before your battery gives up , but again I'm not using a pod so couldn't say for sure

[kamilb1998]

LifePo4 batteries are great. I run my C5 with a 60Ah 12v one made by ping battery and get around 45-50 miles out of it for just 7.5kg of weight (I have it mounted in the boot ).

Just out of interest, how much did your battery cost, if you don't mind me asking?

[alecjw]

Just out of interest, how much did your battery cost, if you don't mind me asking?

Would also be quite interested to know! I've been quoted £500 for my 120Ah worth of batteries.

Dan: thanks for your reply. Yep, I was thinking it might completely screw over the battery level meter, in which case I may use that as an excuse to replace the pod electronics with a microcontroller, and use the ADC with a lookup table for the LiFe discharge curve.

[dansretropod]

Off the top of my head it was about £400 (inc the BMS & 10amp charger).  So you getting 120ah for £500 seems to be a bargain   They ain't cheap, but the range you get out of them plus they can be recharged 3x(?) as many times as lead acids makes direct cost comparisons difficult 

I'm wondering now what sort of Ah value you need in a car battery to do 45 miles?  Sinclair's original claim of 20 miles from a 36ah battery were ludicrous (well unless you didn't use the motor much )

[kamilb1998]

Off the top of my head it was about £400 (inc the BMS & 10amp charger).  So you getting 120ah for £500 seems to be a bargain   They ain't cheap, but the range you get out of them plus they can be recharged 3x(?) as many times as lead acids makes direct cost comparisons difficult 

I'm wondering now what sort of Ah value you need in a car battery to do 45 miles?  Sinclair's original claim of 20 miles from a 36ah battery were ludicrous (well unless you didn't use the motor much )

Price wise they work out about the same as lead acid batteries as lead acid batteries need replacing quite often.

I get about 12 miles of motor time from a 35Ah battery.

[alecjw]

Off the top of my head it was about £400 (inc the BMS & 10amp charger).  So you getting 120ah for £500 seems to be a bargain   They ain't cheap, but the range you get out of them plus they can be recharged 3x(?) as many times as lead acids makes direct cost comparisons difficult 

I'm wondering now what sort of Ah value you need in a car battery to do 45 miles?  Sinclair's original claim of 20 miles from a 36ah battery were ludicrous (well unless you didn't use the motor much )

Lead acid batteries are strange beasts. The original battery was 36Ah but by peukert's law looked more like 20-25Ah. Then if they're standard car batteries, you don't want to discharge by more than 20% of capacity, so that gives you about 4-5Ah effective. If they're deep cycle, you can probably get away with discharging them to 50% reguarly (I think this is roughly what the C5's low voltage cutout limits you to), so that's 10-12Ah. Because of peukert, your range would go up superlinearly with battery size, but I think you'd soon reach a point at which the extra load from the battery weight balances out the extra capacity!
Weight would make a significant contribution to battery life - when I'm going along on the flat, my motor load is at the second green marker. I weigh 80kg. I let a friend, who weighs 100kg, have a go in mine and for him the motor load stayed at the first orange marker.

Also, re durability, I think Pb are typically rated for 500 cycles and LiFe to 2500 cycles. So more like 5x as long. If I was going to try and make a comparison, i'd say I'm getting about 10x the capacity - but to allow for the possibility of my calculations above being bull*pardon my French*, let's say 5x. Then I have 5x as many cycles. If I went to halfords, a new lead acid battery would probably set me back £50. If I say utlity is linear in range and in number of discharge cycles, i'm getting 25x the utility for 10x the cost, so I'd say it works out quite a good deal!
Actually, if I don't run the batteries into the ground, they'll last even more cycles than that. The datasheet says 2000 cycles at 80% discharge and 3000 cycles at 70%. I expect I'll rarely discharge below halfway, so perhaps I could get a very large number of cycles indeed...

Dan: what charger and BMS do you use? I thought I could probably get away without a BMS since LiFe's are fairly hardy anyway, what do you think?

[dansretropod]

5x the capacity, I'll go with that   The charger and BMS were all part of the purchase from ping battery in china.

I didn't realise car batteries were so bad and even the deep discharge ones are not too great either.  Well I know what I prefer: Spend the money up front and get a battery thats probably going to last me 10 years and be able to pedal/motor my c5 up inclines/hills a hell of a lot easier than having a 20kg battery weighing me down