question

northcaptain avatar image
northcaptain asked

Multiple battery chemistry connected to Smart BMS

Looking at the datasheet for Smart BMS 12 200, the start battery and alternator are connected to the Alternator Start Battery + port and it is supposed to forward current to the Lithium battery port and System + only when 13 V have been reached.

But then... what happen to that poor battery since as soon as 13 V is reached, there is an alternator that will send the bulk/absorption voltage to both battery which are different. The start battery might be a Victron AGM. They have fairly the same charge profile:

Victron AGM charge profile @25C

Bulk

Absorbtion 14.2 – 14.6

Float 13.5 – 13.8


Victron Lithium charge profile (not temperature related)

Victron Lithium charge profile

Bulk 14-15

Absorbtion 14.2

Float 13.5


Let's suppose the alternator is a Balmar of 250 Amp and the regulator is programmable, and that the AGM are almost full already - they have just started the engine. And assume the basic charging profile will then be adapted for the Lithium. But lets remember that we have 2 different battery chemistry thus interconnected.

For a small daily charge which will not be complete, lets say an hour of alternator or a day of solar panel i don't think there is a problem with the AGM being exposed to 14.2 volt for a long time.

But then it is recommended to fully charge the Lithium at least once a week. So then the AGM will be exposed to 14.2 for many hours. The voltage is 14.2 but the current will be null since the resistance of the Lithium is far less than the AGM. But nevertheless.

What would be Victron guidance for use of both battery chemistry on the Smart BMS given the requirement of a long complete charge of the Lithium that have the risk to expose the Victron AGM to longer periof of high voltage ?

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BMS
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2 Answers
Stefanie (Victron Energy Staff) avatar image
Stefanie (Victron Energy Staff) answered ·

Hi @Northcaptain

That's not how it works. Please take the following into account:

  1. The Smart BMS does not provide any kind of charge algorithm.
  2. Alternator charge stops charging the lithium battery once Battery charge limit (set in BMS via VictronConnect) has been reached and only will resume charging when the voltage at the alternator port drops 0.5V below Battery charge limit.
  3. For a long complete charge of the lithium you could use a "real" charger (MPPT, AC charger, inverter/&charger) at the System+ port. But I think that charging from Alternator up to 14.2V gives sufficient time for the lithium to balance (balancing starts at ~13.8V).

In between, the alternator continues to charge the starter battery as usual via the alternator regulator.

FWIW: there is a new manual. See https://www.victronenergy.com/media/pg/Smart_BMS_12-200/en/index-en.html

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northcaptain avatar image
northcaptain answered ·

Hello thanks for the updated manual i haven't seen it. Understand there is no charge algorithm it all bear on the alternator regulator, and this regulator should be set for Victron LiPo charge characteristic although there is no feedback from the battery to the regulator. Only an ultimate disconnect like you said when the BMS will stop inflow of current on the Alternator Start Battery + port once Battery charge limit has been reached ?

I am on a boat, and all my customers are also on boats. The solar can provide much of the daily energy but there is no space on a boat to have enough solar panel to completely charge a lithium bank (400Ah) that has been depleted to 30%. (deficit of 280 Ah) So we all rely on alternators to do so, since we also have to heat water and make water (very energy consuming). The pattern is to have the engine running once a day for 1 hour. With a large alternator, you cant get 150-175 Ah in an hour. And the remaining charging would be done by solar panels.

I agree that charging with an alternator someone has to stay awake and check all the time to make sure the charging voltage is ok.


But my question was related to having a single charging profile (the alternator) and two battery chemistry. It would be the same question if an alternator was feeding a Lithium and an AMG through an Argofet. How to make sure the AGM is not overcharged while the Lithium get its 14.5 volt of charging voltage. Once the AGM is charged, what happen if we continue to supply voltage to it ?(the voltage present at Alternator Start Battery + to feed the lithium) ?

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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ commented ·

But that's exactly how a "normal" alternator works. Depending on the integrated regulator in the alternator (they range from roughly 13.8V to 14.6V), they keep the voltage of the starter battery at this level (that's one reason why some people prefer still using a battery isolator ala Argofet to drop the voltage a bit) as long as the alternator is running (and regardless of whether the lithium battery is being charged at the same time). Lead-acid handles this quite well, but not an ideal situation, I agree.

I handled this differently on my sailing yacht by using a Lynx Smart BMS. A high power alternator is charging a 400Ah lithium bank directly, controlled by a Wakespeed WS500 regulator. The starter battery (Victron AGM 110Ah, only for starting the engine and powering the bow thruster) becomes charged by a Orion-Tr Smart 18A charger.

Admittedly not exactly a cheap solution, but the optimal solution for both chemistries, since each has its own regulated charging algorithm. And it's efficient.

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northcaptain avatar image northcaptain Stefanie (Victron Energy Staff) ♦♦ commented ·
Nice setup ! wakespeed is the way to go. I have a Balmar 250 A with a MC 618.. for the moment. Agree that each battery should have its on regulation profile. I guess i was distracted by the Smart BMS 12-200 simplicity (no need for DC to DC charger) but I don't understand what Victron had in mind in having a common interconnection port for both chemistry of batteries. In fact the 12-200 BMS has an internal battery combiner so it really interconnect the AGM with the Lithium while the charge is in progress. I think i will have to program the MC 618 carefully. I have ordered 4 x 200ah lithium and they have not arrived yet - i'm at the planing stage.
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Stefanie (Victron Energy Staff) avatar image Stefanie (Victron Energy Staff) ♦♦ northcaptain commented ·

I think the idea behind the Smart BMS 12-200 is to provide an easy to go solution which combines a few devices into one single device - Current Limiter, Battery Combiner, Battery Protect and a BMS, which safes a lot of extra installation effort and extra devices.

The Smart BMS is certainly not the ultimate solution for all systems, but it has its raison d'être.

The fact that there are two different chemistries is irrelevant, since an alternator does not provide a charge profile for either one. It is nothing more than a generator that supplies current in a certain voltage range.

Good luck with your next installation!


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