# [EVDL] Need to design BMS system for LiFePO4 cells - some questions



## EVDL List (Jul 27, 2007)

(this is a repost via nabble - I was not subscribed to the list when I first
posted)

Hello.

I need to design a BMS system for LiFePO4 cells. I understand the
monitoring part completely - linking the BMS system to the controller to
prevent over-discharge and to the charger to prevent over-charge. My
question is about balancing the cells. A simple way that comes to mind is
to simply discharge the cells (FET transistor + power resistor for each
cell) until all the cells have about the same voltage. However, it would be
nice to be able to take fully charged cells out of the series chain during
charging. No simple way to do this comes to mind. Am I missing something
-- are there some clever tricks I haven't thought of? Relays or other
switches that bypass the cells are out of the question for my design.

Thank you,

Fran

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## EVDL List (Jul 27, 2007)

Hello.

I need to design a BMS system for LiFePO4 cells. I understand the
monitoring part completely - linking the BMS system to the controller to
prevent over-discharge and to the charger to prevent over-charge. My
question is about balancing the cells. A simple way that comes to mind is
to simply discharge the cells (FET transistor + power resistor for each
cell) until all the cells have about the same voltage. However, it would be
nice to be able to take fully charged cells out of the series chain during
charging. No simple way to do this comes to mind. Am I missing something
-- are there some clever tricks I haven't thought of? Relays or other
switches that bypass the cells are out of the question for my design.

Thank you,

Fran
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View this message in context: http://www.nabble.com/Need-to-design-BMS-system-for-LiFePO4-cells---some-questions-tp14845214s25542p14845214.html
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## EVDL List (Jul 27, 2007)

This doesn't sound correct at all. Voltage is probably not a very accurate way to determine charge state.

Then if one cell is say 10% lower capacity just because it was manufactured on a different day, and they all start out at the same charge state, it will suffer an additional overcharge as they all reach full charge. That overcharge will likely further reduce the capacity of the smaller cell.

It's not exactly and efficient or practical method either.

Do you have a link to the mfg's charging specs? AFAIK the job requires a constant string current, then an accurate shunting to guarantee a constant voltage state with current measurement, then a shutoff mode where all current is bypassed away from the cell. 

Danny



> ---- fsabolich <[email protected]> wrote:
> >
> > (this is a repost via nabble - I was not subscribed to the list when I first
> > posted)
> ...


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## EVDL List (Jul 27, 2007)

Since I would be monitoring each cell's voltage during charge, the cell with
10% lower capacity wouldn't be overcharged; charging would simply stop when
that cell reached 4.2V. Basically the higher capacity cells wouldn't get a
full charge.

Anyways, I have decided to change the method a bit - the hardware would
still be the same. As soon as one cell reaches 4.2V charge current is
reduced. As each cell reaches 4.2V its FET is activated to parallel its
power resistor. The current flowing through the power resistor is equal to
the charge current, effectively bypassing the cell.

This is not very efficient and can take some time to balance the cells, but
it is relatively simple.

Does this method sound better or are there other problems with it?

Fran




> dannym wrote:
> >
> > This doesn't sound correct at all. Voltage is probably not a very
> > accurate way to determine charge state.
> ...


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

[No message]


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## EVDL List (Jul 27, 2007)

> [email protected] wrote:
> > This doesn't sound correct at all. Voltage is probably not a very accurate way to determine charge state.
> >
> 
> ...


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## EVDL List (Jul 27, 2007)

I was sent a very helpful PDF. You can find it by googling "a review of cell
equalization methods" - should be the first hit.

Active cell balancing using multiple transformers doesn't seem that
complicated. If I can find suitable transformers I may even consider that
method, otherwise I'll just use a MOSFET + power resistor scheme.

The PWM idea has potential. I suppose you could have a learning algorithm
that PWMs certain cells to slightly reduce their average charge current so
that they all finish charging at the same time (or as close as possible to
it).

Have you given active balancing (pumping energy around) any consideration?

Fran




> Morgan LaMoore wrote:
> >
> > I'm planning on doing something similar. You can even PWM the power
> > resistor for better control of the shunt current.
> ...


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## EVDL List (Jul 27, 2007)

What about bank charging LiFePO4's?

Say you had a 150V (50 or so cell series) pack. Each parallel group has 
a 20A single-cell LiFePO4 charger. How much would that cost, compared 
to a BMS and charger? (A OTS 2A charger is like $15, so maybe the 20A 
version wouldn't be cheap.) You'd have to limit discharge, you wouldn't 
be able to go to 0% DOD, but would 20% be okay to prevent reversing or 
exploding anybody?

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## EVDL List (Jul 27, 2007)

> fsabolich <[email protected]> wrote:
> > I was sent a very helpful PDF. You can find it by googling "a review of cell
> > equalization methods" - should be the first hit.
> 
> ...


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## EVDL List (Jul 27, 2007)

> Morgan LaMoore <[email protected]> wrote:
> > On Jan 16, 2008 10:02 AM, fsabolich <[email protected]> wrote:
> > > I was sent a very helpful PDF. You can find it by googling "a review of cell
> > > equalization methods" - should be the first hit.
> ...


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## EVDL List (Jul 27, 2007)

I don't see how you'd meet lithium's charging profile with a flying capacitor.

As far as I understand it you're proposing using an isolated transformer winding to provide a charger for one or at most several cells? Well, remember a transformer winding is not a charger in itself. It needs a rectifier, filter, and a regulator, most likely a dc/dc converter. That gets expensive to repeat many, many times for all the cells. It gets rid of the need for high-power resistors for bypassing in the constant-voltage or shutoff stages IF each cell has its own charger. 

If however we break it down into a 10A charger winding for each say 4 cells, we still have to add these bypass resistors capable of handling the 10A (around 40W) and a charge regulator for each cell. It's more efficient than handling the whole string at once because once a section of 4 cells all come up as "full" we can turn off the whole charger for that section and nothing needs to be bypassed. 

If I were to do it, I'd propose that say we have one say 10A smart-charger supply communicating with the cells. Once one cell comes up as "finishing charge" and going into Constant Voltage mode by switches on its bypass (shunt) resistor. Now in order to avoid having to put a 40W resistor on each cell (which is a HUGE amount of heat!) we do like 10W resistors and as the most full cell reports more and more bypass current to maintain that Constant Voltage it will reduce the overall string current to stay within that 10W limit (ultimately like 2A). The other cells may need to be informed of the lower current to know when to switch to this CV stage- or not- I'm not sure what this tech of batt's requirement is here. Need a mfg spec sheet.

The great thing is that with each cell at 3.3v or 3.6v or whatever, we can probably put a microcontroller on each one without a regulator. A microcontroller can usually take a surge up to 7v or so without getting messed up but of course that issue would require some review. Not needing a reg is a good thing. They're expensive and most cheap ones have a significant quiescent current always draining the cell.

Danny



> ---- Morgan LaMoore <[email protected]> wrote:
> > > Have you given active balancing (pumping energy around) any consideration?
> >
> > I've done calculations of efficiency and speed of capacitor shuttling.
> ...


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## EVDL List (Jul 27, 2007)

Fran,

I have done similar design for one of OEM customers of mine - few years 
back. It works, if you get details right.

You can see few photos of such design for Kokam cells as an example:
http://www.metricmind.com/bms.htm You can see SMT power resistors
on the PCBs. I wouldn't necessarily design it this way, but
I was constrained by the available space and this is how my
customer wanted it done.

Cell Voltage is fairly good
indication of SOC for one type of Li cells and no indication at all
for other (like A123 for instance. They have identical voltage
at 20% and 70% SOC). You need to study manufacturer's data, then
test the cells yourself. If you shunt a cell with resistor and FET switch,
while on charge trying to save time and come up with identical
voltages at the same time, you don't know the current value you're
shunting, so you can't accurately keep track of SOC of each cell.
You need constant current sinks, not resistors if you want real
BMS using this concept, not poor man's BMS. But you may not care,
poor man's BMS which doesn't keep track of SOC's of individual
cells may well be "good enough", depends on what you (or your
customer) requires. IF it safeguards the battery from
over-anything, that might well be sufficient.

Metric Mind Corp (my company) plans to release similar design for 
general public to be used with LiFePO or any other Li chemistry
cells - something I for various reasons avoided doing for
some time. I only have to sort out software ownership issues first,
I'm not a programmer so I subcontract out this part of BMS development.

Good luck with your project, let us all know how it turn out,

Victor
'91 ACRX - something different




> fsabolich wrote:
> > I was sent a very helpful PDF. You can find it by googling "a review of cell
> > equalization methods" - should be the first hit.
> >
> ...


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## EVDL List (Jul 27, 2007)

Thanks for the info and advice.

As for voltage versus charge state, I checked the spec sheet and there does
seem to be some slope for 80% to 20% charge. The cells I ordered are
TS-LFP90AHA (from Everspring). This is not my hobby so I don't have time to
put together a pack from A123s for example. I know you personally (and
others) have been burned by ThunderSky, but I looked around and couldn't
find anything comparable. Anyways, this is a can of worms I don't want to
open in this thread. I will let people on this list know how they perform. 
The first order is small, and if they burn me then they loose subsequent
larger orders.

The first vehicle is small and simply a test of concept, batteries, and our
electronics. So far I have been busy with the motors and controllers
themselves; I just started learning about the batteries, BMS, etc.

I will let you guys know about my progress when I have something worthwhile
to post.

Thanks,

Fran




> Metric Mind wrote:
> >
> > Fran,
> >
> ...


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## EVDL List (Jul 27, 2007)

My regulators work like this: When the battery that they are connected 
to gets to the proper voltage they shunt the charge off to a high 
wattage resistor instead of thru the battery. This does waste a bit of 
the charge but it keeps the battery from being over charged while it 
allows the rest of the pack to come up to that same level. Of course I 
have 6 volt lead acid batteries at this time but I don't see any reason 
why the idea would not work with any other voltage or type of battery. 
I have a regulator for each one of my batteries and when I see most of 
the regulators flashing (the are equipted with a led and a lamp blub) 
and the charger is putting out minial amps I know the pack as balanced 
as they can get. I can eaisly check with my volt meter to confirm 
that all are at or near the same level.

Lloyd Wayne Reece
1981 Lectra Centauri
Las Vegas, NV





> fsabolich wrote:
> 
> >Hello.
> >
> ...


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## EVDL List (Jul 27, 2007)

> Lloyd Wayne Reece wrote:
> > My regulators work like this: When the battery gets to the proper
> > voltage, they shunt the charge off to a high wattage resistor...
> > instead of thru the battery. This does waste a bit of the charge but
> ...


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