question

taede avatar image
taede asked

REC BMS / Cerbo GX / Multiplus II absorption and bulk charging method?

My question is: when does the charger change from absorption to bulk charging and how domI ensure that the battery is not trickle charged to death. I have contacted REC BMS and had quick replies, but I am not able to resolve my issue.

l run a REC BMS, Cerbo GX and Multiplus II 48/3000/35 on a 16S 300Ah LiFePO4 battery pack on my electric boat with all the components connected via CAN. The BMS controls the charger/inverter via DVCC and I share the current and temperature sense from the REC BMS.

I started using the default settings from the BMS and I adjusted some paramters afterward, like capacity, max charge and discharge current.

The parameter: cell end of charge voltage is set to 3.58V which is good

The parameter: end of charge hysteresis per cell is 0.25V which would be good (3.58-0.25=3.33V) BUT somehow REC BMS factors this 0.25V with 0.2 ( see latest REC Q BMS manual, which means that the battery floats at 3.58-(0.25*0.2)=3.53V per cell … which is in my opinion REALLY BAD?! for LiFePO4 cells…

The reaction from REC BMS:

I checked again and you are right, the factor 0.2 is used. It is because better MPPT efficiency when the battery is charged, more energy from the sun is used. This was developed together with Victron recently.
SOC hysteresis cannot completely switch off the charger, it only limits charging current for chargers.


So why does Victron wants these cells to be trickle charged to such a high voltage…

To resolve the high trickle charge voltage I changed the parameter: end of charge hysteresis per cell to 0.9V instead of 0.25V. Because 3.58-0.9*0.2=3.4V which I would think is a better float voltage (should even use a lower voltage). I have done this in concert with REC BMS support.

However, now my charger stays in float and chargers the battery to 54.4V (3.4v/cell) even when the battery voltage is lower then 54.4V when the charger is switched of.

I wonder when does the BMS decide to charge the battery to 57.3V (3.58V/cell) again? Is that the case when the battery voltage drops below the end of charge voltage-hysteresis which is in my case 3.58V-0.9V= 2.68V?

The parameter: end of charge SOC hysteresis (default 5%) does not appear in the settings tab when using the REC BMS WIFI module. Not sure what this does.

What i have seen is that my battery was at 53V (below the float voltage) with the charger off and at 83%. The DVCC CVT voltage (charge voltage) is still 54.4V (3.4v) and not 57.3 (3.58v)

So how do I charge my battery to 100% full when I want that before going on a boat trip, how do I ensure that the battery is not trickle charged to death?

I could reset the hysteresis to 0.25V again and this will probably recharge to 3.58V when the battery voltage is below the 3.58v-0.25V, and switch of the charger after charging is finished, but this will not work when I connect the solar array in the near future. So I assume in this case that the bms commands to bulk charge the battery voltage is below the hysteresis voltage, but i am unsure if this is how it works.

Why the 0.2 factor on the hysteresis? The BMS charge method would be OK if the factor was not used. The battery would then float at 3.33V instead of 3.58V and the bulk charge would start again when the battery drops below 53.3V (3.33V/cell). The SOC end of charge hysteresis of 5% would ensure that the battery only recharges when the SOC is below 95%!

THIS WOULD BE PERFECT I THINK, but the 0.2x factor ruins the whole charge algorithm and the 5% SOC hysteresis can not be set anymore, and perhaps also not used in the algorithm anymore.

I am the edge case as some people in this forum would call it, but I think the BMS is totally useless to me for charging the battery when the BMS will destroy the expensive battery by trickle charging it to death.

I wonder what
- other users experience is,
- whether some of my findings and ideas are actually correct.
- hot to get a working system, ideally as described above

BMS
2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

7 Answers
taede avatar image
taede answered ·

After consolidating the issue with REC BMS, I received a firmware update.

The default 0.2 factor as mentioned in the post is now changed to 1.0

I also confirmed with REC BMS that the 5% SOC hysteresis is implement, meaning that recharge only starts after at least 5% of the capacity is drained (less then 95% SOC)

This is how my BMS with factor 1.0 instead of 0.2 (default) now works:

I set the end of charge voltage hysteresis at 0.25V. The battery now floats at 3.33V/cell and the bulk charge starts when the voltage drops below 3.33V/cell (53.3V 16S battery voltage). The SOC hysteresis of 5% ensures that the battery only recharges when the SOC is below 95%!

You can read/set the SOC hysteresis with the WiFi module via menu: firmware / manual and use the manual command: SOCH? or set a other hysteresis via command SOCH x.xxx (5% = 0.050)

1 comment
2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

chris-bme avatar image chris-bme commented ·
I have recently installed a REC BMS and am having the same issue you described in the original post. Was the firmware sent to you a one-off version or is this now the latest version? Is this modified firmware available for download anywhere?
0 Likes 0 ·
jokani avatar image
jokani answered ·

Hi Taede, two very interesting posts, I am just finalising a REC BMS install, and have been struggling to get how the charge/float settings as I want.

Is this the firmware version you are now using:

bms_1A_Victron_v2_6.bin

Good to know the SOCH command, I'll add that to my notes.

I have just amended my system and put 2 Lead AGM batteries in parallel with the lithiums, early days but seems to be working OK at the moment. Now at the end of the sailing season I can get the lithiums down to 50% SOC and switch off the REC BMS, and leave the AGM's to to work in the winter when the boat will be permanently on shore power.

2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

multiplex avatar image
multiplex answered ·

Hi Taede


Can you pls share this firmware update I’ve got the same issue




1 comment
2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

baxter avatar image baxter commented ·
Hi, the factor is configurable with firmware 2.8. pls get in contact with REC as firmware is bind to an internal id of the hardware.
0 Likes 0 ·
basm avatar image
basm answered ·

@Baxter you mean the factor 0.2 to 1.0 in the charge algorithm?


I send REC an email about two weeks ago. No response as of yet.


Kind regards.


Bas

1 comment
2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

baxter avatar image baxter commented ·
Yes, since 2.8 this parameter can be configured.

The response time is rather long, send them a reminder and include the BMS name.

0 Likes 0 ·
multiplex avatar image
multiplex answered ·

I have send an email to Tine waiting new firmware

2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

multiplex avatar image
multiplex answered ·

I am ready they send me a new firmware

Everything works perfect now

2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

basm avatar image
basm answered ·

For others: Received the new firmware as well. Specifically bms_1A_Victron_v2_7.bin.

2 |3000

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.