Hi there,
I recently upgraded my Narrow Boat to a lithium setup with your system. My setup includes 3x 200Ah Lithium batteries, protected by the BMS CL 12/100 BMS. I have a Multiplus 12/3000/120 inverter/charger, with smart battery protection protecting load and solar charge (Bluesolar 100/30). My engine is a Betamarine 38, with separate leisure and starter alternators. I have wired an old wet lead acid battery as a dump battery in parallel to my alternator (kept from my original wet battery setup), so as to protect the alternator diodes from voltage surges in case of any shutdown from the BMS whilst the engine is running, as per the advice in the user guide. The whole set up is fused fully as per the installation guidelines and all works really well, I have the setup running for 6 months with no issues, however..
The boat is moored at a local marina, sometimes for over a month without the engine being run (the solar keeps the lithium in tip top charge). Whilst I could connect her to the shoreline via the Multiplus, the solar keeps the lithium charged enough to leave the fridge on etc. The problem I have is that the dump battery has flattened over time, and the voltage drop across it is 10.5v now - I suspect this is because the BMS has slowly over a month or so flattened the dump battery combined with the natural discharge/flattening over time. I'm not sure what the minimum voltage is before the BMS switches off and in turns loads and solar charging off.
My question, is if there is some way of doing away with the dump battery? From what I can see, the dump battery serves 2 purposes - it independently powers the BMS when the engine is not running, (14ma is what I seem to recall), but also protects the alternator in case of a BMS disconnect whist the engine is running. The latter I can solve with an alternator protect, (having to go to another manufacturer for this), however the former, I cannot see why I could not power the BMS by the lithiums when the alternator is not running. This could be done with some simple diode arrangement, allowing a small amount of current to flow from the lithiums to power the BMS when the alternator is not running, when it is, the diode will prevent current flow directly back into the lithiums from the alternator and ensure all the power goes via the current limiting capability of the BMS back to the batteries. I can't see why this would not work - I'd include a low current fuse in line, (the maximum current flow should be no more than the drain of the BMS), and would take the feed from the load side of my load battery protect, ensuring if the bms tells everything to shut down, then it will, in effect shut itself down, which is fine as doing that would shut down all battery protect and charge systems anyway. To start the system up again in that scenario, I would just need to start the engine up, or have a switched connection from my starter battery to the alternator post on the BMS to fire her up again and in turn bring the load side of the battery protect back on again. I have a diagram, but it's only part finished will finish and post if it helps.
If you can tell me if this setup might work, or indeed if there is any other arrangement you could suggest.
My starter battery is charged by a dedicated separate alternator so is not a factor.
Any thoughts / suggestions very much appreciated.
Howard.
