Re: Alternator charging a Lithium bank

Scott SV Tengah

Hi Doug,

Seems you read what I wrote but it didn't provide the answers you needed? Probably the result of someone who studied economics trying to explain something way out of his pay grade. :)

I'll try to answer your questions, though.

- As Dean alluded to, you do not want to base your alternator charging cutoff on battery bank level voltages. Your Victron chargers, interfaced to your Victron BMS will do that automatically. The Quattro does it through ethernet but my Skylla-i 24/80 needed an interface wire, which of course Victron sells. I have seen my Victron BMS, which monitors cell level voltage, cut off charging while the bank voltages and my BMV-712 (which I think is a coulomb counter?) showed it was not even 90% SOC and the bank level voltage seemed fine at around 28.0v. The voltage spike on the one problematic cell was hidden by the other (4x6-1=23) cells that were not spiking.  As an aside, this imbalance occurred because I didn't charge to 100% for a few months and small cell imbalances started accumulating. That was resolved with few hours at 100% with solar and a temporarily extended (4 hr) absorption time, which allowed the batteries to self-balance at a cell level.

I cannot remember exactly where I set the alternator absorption voltage to, but I recall it being somewhere around 27.4v on the Alpha Pro II. This makes it such that my lithium batteries are never fully charged by the alternator, but I'm ok with that, especially when I find myself unhappily motoring for 24 hours in the doldrums! The 27.4v usually represents anywhere from 80-92% bank SOC - again a reminder that you can't rely on voltage to represent SOC on lithium.

I assume you wanted to install the Cyrix relay to abruptly disconnect the alternator from the battery bank if the BMS detects a cell overvolt/overtemp condition? I would not suggest that as it could damage the alternator. Maybe not, but why take the risk when you can simply use the Ve.BUS BMS "charge disconnect" wire to control the relays as I suggested in this post:

As Dominique stated, the Victron BMS cannot power a normal Hella relay. I tried many times and had senior techs at Victron helping me, but it simply didn't have enough power to do so.  That's why I had to put in a much lower draw solid state relay in combination with Hella relays. See the post I linked for details. It has worked perfectly for 10,000nm over a year of full time cruising since I installed that alternator cutoff system. A real engineer, who built a well known company hacking into car ECU and control systems helped me implement it, so that should give you more confidence that it works. :) The low absorption voltage is an additional protection in case one of those relays fail.

- How do you intend to have your Victron BMS control your Mastervolt battery chargers? I don't have MV chargers, so can't provide advice, but you need to make sure the MV chargers stop charging when the BMS detects a problem. 

- Clarification: the 24v alternator is a Mastervolt too, not a Volvo charger. The Volvo charger (Bosch, really) is the 12v charger and doesn't have anything to do with my lifepo4 system. Get the alt temp sensor and install it on the MV. I have never seen my alternator taper charging current due to alternator overtemp, even when it charged the bank from 30%-90% SOC. That's roughly 3.5 hours at full output on my system. But you can never be too safe if you're in the middle of the Pacific without a MV dealer nearby.

- I did not install the Cerbo, Touch GX or the DMC. I found that I could replicate most of their functions without the additional complication and parasitic power draw of those devices. It's cool stuff for sure, but there is a draw and it's simply more components to fail. 

- Make sure you can turn off your Quattro inverter easily. I think Victron says it draws 30w at idle, but I am seeing more like 45-50w. That's almost 50amps a day of parasitic loss if you kept your inverter on 24/7, equivalent to the power output of a 250w solar panel, more or less! We have a simple switch (see post linked above) that we turn on right when we use it and off when we're done. 

- I use the BMV-712 right now to keep the batteries from being kept at a high SOC by the solar, per the solution I detailed on my previous post. We are sadly away from Tengah for 3 months due to visa issues but the boat watcher tells me the batteries cycle between 65%-85% range as I intended. The bonus is that the dehumidifier I am using to burn off excess solar energy is keeping the boat dry inside. For this possible use and also the reasons mentioned above, I wouldn't use this BMV-712 programmable relay to control your alternator.

Hope this helps clarify things a bit. 

2007 A54 #69
SV Tengah

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