Bluetooth BMS with IR compensation

Batteries, Chargers, and Battery Management Systems.
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avandalen   10 W

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Bluetooth BMS with IR compensation

Post by avandalen » May 23 2019 5:05am

For a small production of solarbikes I need a Bluetooth BMS with internal resistance compensation. I can't find anything on the web.
I charge the 36V LiFePO4 at 8A, without IR compensation this is not possible. Ubattery = U open + Icharge * RI. There has to be compensated for Icharge * RI.
I have no time to develop my own BMS.
Maybe we can modify an existing BMS. That would be interesting for everyone, who wants to help?
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https://www.avdweb.nl/solar-bike/electronics/bms

BotoXbz   10 W

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Re: Bluetooth BMS with IR compensation

Post by BotoXbz » May 23 2019 5:34am


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flippy   100 kW

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Re: Bluetooth BMS with IR compensation

Post by flippy » May 23 2019 3:43pm

why would you care about the IR on a bike with lifepo4 cells?
what specific advantage are you trying to gain? more accurate SoC?

apart from those questions: why would you ever use lifepo4 on a BIKE?
Lithium beats liquid dinosaurs.

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avandalen   10 W

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Re: Bluetooth BMS with IR compensation

Post by avandalen » May 24 2019 3:55am

flippy wrote:
May 23 2019 3:43pm
why would you care about the IR on a bike with lifepo4 cells?
what specific advantage are you trying to gain? more accurate SoC?

apart from those questions: why would you ever use lifepo4 on a BIKE?
Only a LiFePO4 battery is suitable for my solar bike because of the high charging current (sometimes 300W solar power = 8A / 36V)

The need for internal resistance compensation
The need for internal resistance compensationDuring charging, the voltage is measured across the battery, but this is not the actual voltage because the voltage drop across the internal resistor (Ri) is not taken into account. The actual battery voltage during charging is lower than the measured value:
U battery = U measured - charging current * Ri
As a result, charging is stopped too early. The opposite is the case with discharging. Unfortunately, no compensation is done in any BMS for ebikes.

Here is a calculation:
The internal resistance of the battery that I use is 110mΩ (LiFePO4 A123 ANR26650 12s2p). The voltage drop across the internal resistor with a charge or discharge current of 9A is 1V. You can see that this has a lot of influence in the discharge graph:
Image
LiFePO4 battery discharge graph

The majority of the discharge graph is between 37V and 39V, and therefore a measurement error of 1V is completely unacceptable.

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