[EVDL] My BMS

879 Views 2 Replies 1 Participant Last post by EVDL List, Feb 22, 2011

EVDL List

Discussion Starter · Feb 20, 2011

Guy,

Looking for input on my thoughts.

This is my vision of a BMS for a CALB project I am doing that will also
morph into my EV. I am interested in what you think.

1. All analog, Like Lee would do it or because I am an old far, not sure
which.

2. Temp sensor into a comparator for battery over temp trigger.

3. Temp sensor into a comparator on the heat sink for BMS over temp.

4. Load is comprised of either a FET or a transistor controlling a load
resistor, full on is 5 amps but it ramps from 0 amps to 5 amps from 3.63
volts to 3.67 volts.

5. If any BMS load is active a signal is sent to the charger to limit to
3 amps. If limiting is not possible then logic should switch off the high
current charger and switch on a 3 amp charger.

6. A separate comparator that at 3.75 volts switches off the charger.

7. The above #2, 3 and 6 feed back through the same opto.

8. A separate comparator and opto set to 3.90 volts for an emergency
shutdown.

9. All shutdowns are "latched" at the control board.

10. Balancing circuit only draws current from the battery when the charger
is running.

11. An ALL OK signal from each balancing board to the charger control board
that all BMS are active.

12. The signal from the control board is active for HIGH current. This is
for safety. If there is no activity on the control board then the default is
3 amp charge rate.

13. No processor no lock up, I like that the best.

14. All through hole components, no SMT. My experience is that SMT doesn't
do well in a harsh shock and vibe environment. That and my wave solder
machine prefers through hole. J

15. Should be cheap to build.

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Discussion Starter · #2 · Feb 22, 2011

Mark,

- Do you really need to be able to shunt 5A if your charger
immediately limits to 3 when you start balancing?

- What terminates charging? Is it the All OK signal? If so, are all
batts necessarily at the same SoC, since you use different balancing
currents at different OCV's? Or is the variation in current to adjust
for the series resistance of the battery?

- Like Steven said, try to make it such that a broken reg signals an
error. You could even use an active signal that toggles or something
instead of a simple logic level (Linear Tech makes a battery monitor
IC that does this).

- I like the backup OV comparator. It means you have no single point
of failure.

Great ideas! Keep us posted
-Ben

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Discussion Starter · #3 · Feb 22, 2011

>- Do you really need to be able to shunt 5A if your charger
> immediately limits to 3 when you start balancing?

No, but it won't hurt to, actually I am really old school coming from the
day when you designed to use 1/3 of the capability of components so it will
be designed to carry 7 or 8 amps but only do 3 amps.

>- What terminates charging? Is it the All OK signal? If so, are all
>batts necessarily at the same SoC, since you use different balancing
>currents at different OCV's? Or is the variation in current to adjust
>for the series resistance of the battery?

Charging is terminated by any one of the 3 or 4 fault conditions and under
normal use, cutback to 3 amps happens when ANY of the BMSs starts shunting
and all charging ceases when ALL BMSs are shunting.

>- Like Steven said, try to make it such that a broken reg signals an
>error. You could even use an active signal that toggles or something
>instead of a simple logic level (Linear Tech makes a battery monitor
>IC that does this).

Plan at the moment is that if any reg has a low signal on emergency stop it
won't run. In other words, Emergency stop is an active low opto. Other
possibilities would be to continuously daisy chain a pulse through the
entire series of regulators to a watch dog on the control board, if it is
missing over a period it is known that a regulator did not repeat the pulse.
Eww I like that, hmmmm.

>- I like the backup OV comparator. It means you have no single point
>of failure.

>- Great ideas! Keep us posted

As I progress I will keep the list posted, remember though - I work really
slowly.

Mark Grasser

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