chrisryan43 avatar image
chrisryan43 asked

BMS 12/200 vs Cyrix-Li-Ct


I have just installed a cyrix-li-ct to charge my LiFePO4 and Lead Acid starter battery as per the cyrix datasheet but have now been warned that i could blow up the alternator as the Lithium battery will run the alternator at full capacity. I have consulted a Victron representative who confirmed this to be true. So i am a bit confused and annoyed why the Cyrix datasheet would suggest this set up without any kind of a warning? Is this genuinely a true concern? Has anyone else run this configuration with (or without) any issues?

I am now thinking that a BMS 12/200 would have been a better option but whilst i can find a datasheet if i search for it on the Victron site it doesnt actually have a product page. Is it still in production?

Many thanks


BMSCyrix Battery Combiner
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kiko avatar image kiko commented ·
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Johannes Boonstra (Victron Energy Staff) avatar image
Johannes Boonstra (Victron Energy Staff) answered ·


The thing is that alternators are not having a high efficacy (50%), so if they work hard they get really warm. This doesnt need to be a problem as long as they run at a fairly high rpm as thats causing the airflow/cooling of the alternator.

So many systems have "only " the Cyrix-li-Ct to charge the lithium bank, but as lithium gets larger it would make sense to limit the outout there,

If the alternator is reasonable large compared to the battery, there is no issue, if the alternator runs at >2000 rpm or higher normally, there is no problem.

So check if the ratio engine/alternator can be such that the alternator has a high rpm even at an idling engine.

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chrisryan43 avatar image chrisryan43 commented ·

With that in mind should the Cyrix-Li-ct (which is specifically sold as a product for charging LFP batteries from an alternator) not have a means of sensing the engine rpm so it can engage/disengage at specific speeds? And/or an external temperature sensor which can be placed on the alternator?

It feels like the product has only been semi-thought through

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esteban avatar image esteban chrisryan43 commented ·

Watching the vehicle rpm would be a big stretch for the device. A complication for wiring as well. I dare say you won't find any battery combiner made that does this. In any case, it's better for the alternator to be protected based on heat than rpm. Overheating is the actual issue.

As I understand it, you can charge maybe up to 300 ah of lithium directly from a typical alternator without concern. Not certain of that number though, it's a rule of thumb I read somewhere.

My vehicle has 150 ah lithium and it's fine. My alternator, as I assume most do, has a temperature managed regulator so that as the alternator heats up, the voltage output goes down, reducing the current.

I believe that the ideal solution when charging higher ah battery banks is to use a DC-DC charger. Sterling makes them and Vitcron is supposed to come out with one soon.

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chrisryan43 avatar image chrisryan43 esteban commented ·

Thanks Estaban, that all makes sense. Your rule of thumb seems about right as i have found the largest draw my 200Ah bank demands from my 70Ah alternator is around 30A

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nbo avatar image nbo commented ·

i am think about a similar solution on our sailboat. We have an 80a alternator and I am planning to have 2 180a LiFePo4 battery's with a small BMS and phoenix smart charger.

Battery will be charged mainly by the Phoenix charger. And occasianly when motor is running by the alternator or if empty on sea after 3-4 days.

Do you think it is same to use a Victron Cyrix-Li-ct 12/24-120A or would it be more safe to use a BMS with alternator port like the CL 12/200

Thanks in advance,


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