Dual aternators and dual smart bms not charging correctly

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silverfish asked Dec 14, '23

Dual aternators and dual smart bms not charging correctly

Installation on a catamaran:

2 stock alternators (115A) one on each engine, 2 smart bms 12/200 connected in parrallel via the Li-ion Batt+ port.

No specialized aternator regulator (internal stock regulator is preserved)

4 smart batteries Lifepo4 Victron in parrallel and 2 flooded batteries for engines start.

The purpose of dual bms is to separate/aggregate the alternators power.

Problem(s):

1. It's not possible to fully charge the batteries; the batteries tension never goes up to 13.50V (70 - 80% SOC)

2. When reading the BMS state via BT I can see 13.78V (and 13.8) for alternator tension and

3. charging current is 66% for one alternator and only 35% for the other, even when the alternators are cold (first 5 min. of charging)

Questions:

1. Is there a problem parrallelizing the BMS; are the two alternaors confuse each other with the battery tension, sensing from the bms?

2. 13.8V for the alternator tension seems adequate for the flooded start battery, but how to to get 14.2, 14.4V for the lithium batteries?

More details:

When on shore power the batteries are fully re-charged at 100% SOC via the multiplus charging function. Multiplus is connected on the system+ port

All the cells are fully equilibrated.

I know the limitations with stock alternators, but before the switch to lithium we had GEL batteries and it was possible to see 100 - 120A charging (during a limitate time, sure).

The alternator fuse on each bms is 100A so expecting 90x2 for some time.

Any suggestions or recommandation?

Thanks

BMS alternator smart lithium

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Answer 1 · 2023-12-14T17:58:38Z

pwfarnell answered · Dec 14, 2023 at 05:58 PM

Are your batteries in one bank or two separate banks. If they are in a single bank how have you connected the batteries to the 2 BMS. I do not think that you can have 2 BMS on a single battery bank.

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silverfish commented · Dec 15, 2023 at 11:22 AM

The 4 batteries are in one bank 12V. The bms cables are daisy chained to the first BMS....

That means only the charges and loads connected to that first BMS are controlled?

And only the alternator connected to the second BMS is protected?

The charges and loads connected to the second BMS are not controlled, apart if I use the charge or load disconnect port from the first BMS?

What is the wiring recommandation with two alternators and one BMS?

Thanks for your comment!

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pwfarnell silverfish commented · Dec 15, 2023 at 11:59 AM

@Silverfish

You have the BMS set up correct for the one alternator and that should be working. The BMS that is not connected to any batteries will not work correctly because it is not seeing any connection on the battery ports it thinks that it has a battery fault and will not charge correctly. There have been several posts about this and I believe that you can fool the BMS into thinking that you have a battery connected by fitting a battery data cable extension (https://www.victronenergy.com/cables/m8-circular-connector-male-female-3-pole-cable-2m) to the 2 ports on the BMS. The BMS should still regulate the alternator as the voltage increases and you should get more power out of the alternator. However, there will be no safety action from the battery for low temperature or high cell voltage when charging is not required. You may be able to take a signal wire from the ATC connection on the "Master" BMS (the one that is connected to the batteries) to the remote on/off terminal on the "Slave" BMS (the one not connected to batteries) so that if the batteries stop charging then the Master BMS will turn off the Slave BMS and stop that charging. This is all going off piste and you need to make sure that the system works properly like this.

I am not sure what the best set up for 2 alternators is now you have chosen the BMS 12/200. Above is one option that works on paper in my mind but there are no guarantees. The proper way which is expensive is to fit something like Wakespeed WS500 alternator controllers to each alternator and then a Lynx BMS or Ve BUS BMS (if you have Victron inverter) that can control the WS500s.

You question the charging voltage, the BMS will drop a bit of voltage so what voltages did your alternators produce when charging your lead acid batteries both at high current and low current, if they never got to 14.2 to 14.4V with those batteries then they will not charge the lithium batteries properly. This may only be down to the lithiums being able to accept higher current and the voltage will rise as they fill up. Especially if you get both alternators running and perhaps fit 125A fuses and watch your alternator temperatures.

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silverfish pwfarnell commented · Dec 15, 2023 at 02:04 PM

Hi pwfarnel, thanks for your very detailed answer!

I think your suggestion for the slave BMS is a really good (and cheap) option. I only have to take care of the charges / loads of the slave BMS because if I use the remote on/off switch I think I can only disconnect on "load disconnect" or "charge disconnect" but not both...

And for the alternators charging voltage I think I have to do some more investigations and suvey, when the alternators are cold or hot, when the rev engine is low or high, etc. I agree with you, the wakespeed or so should be better but for now the installation is done as is, and I have to find "less badest solution"

Thanks again.

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pwfarnell silverfish commented · Dec 15, 2023 at 08:38 PM

In my comments above I have answered based on the Smart BMS 12/200, I now realise you did not day Smart so some bits may not apply if you have the non Smart version.

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