tiistai 11. syyskuuta 2012

If I converted a car

I've been researching my ideal kit for a car conversion. AKL (Finnish Central Organisation for Motor Trades and Repairs) has set some rules for working on electric vehicles. The rules define that working with over 50 VAC or 120 VDC requires you to notify the Finnish Safety and Chemicals Agency (Tukes) and the personnel needs to be formally qualified for the work. These are also the same voltage limits set by law in Finland for electrical work. Therefore I have concluded that to avoid any problems one should stay below 120 VDC in private conversions as well. That way nobody should have a problem with these vehicles and it kind of makes me feel a little bit safer and more confident as well since there are set limits to work with.

120 VDC might not seem like much, but since the arrival of the new grey CA-series CALB cells which seem to sag a lot less than anything seen before (except possibly the A123 pouches which are quite hard to work with) you wouldn't need to oversize your pack to account for the voltage sag. They are capable of over 10 C discharge which means you can get decent performance at below 120 VDC and cell sizes as small as 60 Ah (10 C at 60 Ah cell is 600 amps). At a floating 100 volts and 500 amps you're looking at 50 kW in power which should be quite enough for a small car.

This brings me to my kit. It's not a kit really, but a selection components I'd use if I'd start the project today.


I've been using a couple of KP chargers from evassemble.com for a while now and although both of them have arrived with a broken fan, they are quite affordable and seem to do the actual charging fine. On the smaller one I got first the fan itself was broken and/or mounted the wrong way. The 2kW charger I got later had been slightly damaged in transport which prevented the second fan from spinning. These were easily fixable though. Therefore I'd get the KP-L 3kW 116.8 VDC 22 A charger from evassemble.com.

You might think it's a bit silly to start building a kit with the charger, but a good charger is very important and not as easy to come by, especially if you don't want to break the bank. Charge voltage will also determine your pack voltage and number of cells which in turn will define your motor and controller choices. The other option would be to get the biggest charger you can find with charge voltage over 120 VDC and use a relay to cut charging at exactly 120 VDC. You'd only get one more 3.5 volt (charging voltage) cell in though and you'd loose the CV part of the charge. Remember that usually we're first charging with Constant Current (as much as the charger can do) and then we taper the charge by holding the desired voltage (Constant Voltage) while reducing the amps going in until we're at 3.5 volts per cell. The charger will then cut off the charge when a set low current limit is reached.

Alternatively Rebbl sells an Elcon 2500W charger that you can probably program to your needs.

Battery pack

Now that we know our charging voltage which on the KP-L set for 96 volts nominal is 116.8 volts we can calculate that to get about 3.5 V per cell we need 33 CALB cells. It's actually 3.54 V per cell to be exact, but close enough. 34 cells would put us below 3.5 and 32 above 3.6 V, so 33 is the number for us. The reason we are so careful to choose and match our charger with our battery pack is that we will be bottom balancing the pack and just simply charging to full voltage with no BMS. If you were using a BMS you could be a bit more careless, but you'd end up with a mess of wires each a potential fire hazard and we really don't want to go there. Bottom balancing is the way to go and it really does work.

The number of cells is 33 and the rest depends on the size of your car and probably even more on the size of your wallet. I'd say 60 Ah cells are the minimum to get a 40 km range (or 6 kWh and 3000€) and going up to 180 Ah should give you 120 km range in a 200 kg pile of batteries (or 18 kWh and 9000€). If was to order batteries now, I'd order from ev-power.eu.


This one's easy. Or hard. Depends on if you just go with EVnetics Soliton Jr. or decide to find some other alternative. You can order one from Rebbl. They also ship them EMC certified to fulfill all EU regulations. 600 amps should be plenty and if you like more you can always opt for it's bigger brother, the Soliton 1.


This is going fast. Also from Rebbl, or wherever you want to order, the Kostov 10". According to it's spec sheet 115 volts, 270 amps continous at 4500 rpm should give you around 26.2 kW. We don't quite have 115 volts, but close enough to get a ballpark figure. Around 25 kW is very realistic. For peak throughput with the Soliton Jr. at about 100 volts we are looking at double that at 50 kW. Enough for a compact car I think.


There you have it. The main components of a possible future "kWkasi". Pronounced "koo-vee-kasi" in Finnish. Get it? No? Don't worry. Here's the shopping list with quick specs for each component.

Kostov 10" motor (25kW continuous, 50kW peak in our setup, 69 kg)
Soliton Jr. controller (450-500 A continuous, 600 A peak, up to 340 V, 7 kg)
33pcs CALB CA180FI cells (96 V nominal, about 110 V charged, 19 kWh, 185 kg)
KP-L 3kW charger (22 amps at 116.8 volts for 8 hour recharge from empty, 9 kg)


Please note that although I've mentioned Rebbl webshop quite a few times I have not ordered from them anything yet. Which just means I can't vouch for them. They seem respectable though. evassemble.com and ev-power.eu have come through for me just fine on my previous orders.

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