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Discussion Starter · #1 ·
I proposed this idea in the discussion on BMS design techniques, but I think it deserves its own thread. My idea, essentially, is to make a module consisting of cells in series to produce perhaps 1.6 kWh at 160V (10 Ah), so they may be connected in series and/or parallel for up to 640 VDC and any Ah rating needed.

Each module would be self-contained, with an integral BMS and protection against short circuits, overloads, overcharging, and critical low cell voltage. It would have the ability to disconnect itself, if needed, but it would also allow connection to an external charger or controller to take appropriate action.

It may also have a built-in charger, which simply plugs into a standard 120 or 240 VAC receptacle, with either fast (1 hour 1C) charging at 1.5 kW (120V 13A), or overnight charging at 1/10 C. This may be constructed using a capacitor/rectifier voltage multiplier circuit, which is inexpensive, lightweight, and inherently current limited. Its disadvantage is lack of line isolation, but each battery pack would have an internal disconnect which isolates the pack from its external terminals, so it would be completely isolated during charging, as well as when the vehicle is shut down, or upon a crash or other emergency event.

Here are the details (which are suject to discussion and modification):
  1. DC rated fuse (20A 300-1000VDC) for ultimate protection. About $10. Like this:
  2. 20-50A, 600-1200V IGBTs for pack disconnect. 2 x $3.00 = $6:
  3. Electromechanical relay for pack isolation (not for DC switching under load), 20A contacts, 12 VDC coil, 2 x $3 = $6:
  4. BMS master board, with USB or CAN interface, about $50.
  5. Cell BMS units, 50 x $3 each = $150.
  6. LiFePO4 cells, 3.2V, 10Ah, 50 x $16 (at $0.40/Wh) = $800
  7. 1.5 kW charger: $100
  8. Enclosure, terminals, connections, and hardware: $100
So the total cost for a 1.6 kWh module is about $1100, or $0.70/kWh. About 75% of the cost is batteries, and the design could be adapted for larger or smaller cells, depending on requirements. It also incorporates an integral charger, and safety interlocks, and BMS, so those costs are eliminated from the overall build cost. A 19.2 kWh vehicle pack would be 12 modules at a cost of $13,200. I'm not counting engineering expenses, assembly cost, labor, and profit (assuming a commercial product), since this is a DIY kit. But even allowing a 3x cost for sale price, excluding batteries, the total cost would be about $2000 per 1.6 kWh module, or $24,000 for a 19.2 kWh system.

These are ballpark figures, of course. Costs would be lower in high volume or for DIY kits. Using Li-Ion or NiMH would cut the battery cost in half. I think this may be a viable option, and I plan to pursue it with my tractor project in mind. What think ye? :)

· Registered
482 Posts
I like it. Especially since some of lower amp hours cells are pushing enough amps to drive at full power for short distances. A path for scaling up the battery depth using parallel packs can make it easier on the upfront costs of a diy build. Just add more packs to increase range as needed.

I am working on a low cost cell monitoring circuit myself with this type of configuration in mind.

Perhaps you should of kept this topic in the battery forum section.


· Registered
234 Posts
I like the idea of an integrated BMS, but I think it would be a better idea to have the charger in a centralized location.
The charger only needs to connect to the positive and negative most terminals of the battery, so it can easily connect to the battery terminals on the controller.
Having high power electronics which are susceptible to weird problems and big explosions - physically attached to a battery - seems like a bad idea to me.
Not to mention the cost of having a centralized charger would be significantly cheaper (i would guess).

Modular batteries = awesome! I have built my own modular A123 batteries, about 1.58 kwh's each so they are similar to what you would want to build.
A BMS designed for the specific modules would be handy as well.

Here are my modules:

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