locked Re: Batteries starting to get weak - MMM - What to do -

Scott SV Tengah

Some of my thoughts, original in regular font and my response in italics


As a thought - I also am considering if purchasing the SM2k doing a separate 12v bank purely for electronic usage and anything 12v. The Beneteau 57 has a 280amp hr bank for that and separated the 24v bank. Haven't done the calculations for that completely but would dedicate 1x solar panel for keeping that topped off (engine alternator when running also) and be able to charge cameras, flashlights, and run the depth and wind MFd's all day everyday without draining any of the main battery bank. Might be a bit over the top, but it would help. Options for extending the banks could be to put 2x extra 24v bank batteries in the aft clothes locker beside the bank or put them beside the nav station on the floor. Putting that extra pair should be fine as long as all the wires running to the unswitched distribution block from all the pairs are of equal length. That way they will all charge and discharge the same.  

I wouldn't bother with a separate 12v bank. It adds complication without additional benefit. You are at anchor 80-90% of the time anyways, so your bank is really about running house appliances. I don't know anyone who has said that the 24-12v converter can't handle their 12v needs. Further, if the SM2k is anything like the A54, you won't need to add extra space for batteries. I have 450AH @24v and use part of the battery bank area to store unused blankets. Keep in mind we went from 12 gel batteries to six Victron Smart Lithium 150AH@12v. I could have easily added more.

Solar panels - I also noticed MOST people, lets say having 3x panels, have one MPPT to charge the bank. That one MPPT is the same cost as what 3x MPPT's would be for each panel due to inputed power levels. So if you had 3x MPPTs you would have complete redundancy if one went bad. Same cost. 

Please explain what you mean by 3x would cost the same as one due to inputed power? Are you saying that you gain so much cumulative AH due to more shade tolerance that it's worth it to pay for 3 MPPTs? 

 If we all could run the AC unit or units off the bank I am sure we would. However take a look at the Gone with the Wynns website and on there they have a video explain a "soft start" modual that attaches to the ac unit and will drastically lower the amp hrs used and keep the kick start amp hrs way down. It manipulates the start up sequence to prevent the spike. This possibly if utilized would allow more usage of the AC's in a hot climate. I will be installing at least 1 if not for all three units. 

I thought a soft start only impacted the current drawn at compressor startup? How does it impact current draw once the AC is running? We can easily start and run our AC units on our 5kw Victron inverter and unless Climma includes a soft start module, we don't have one.

Either custom setup or OEM drop ins with the available space I do want to maximize the amp hrs usable. I will be a heavy user for sure and don't want to run the gen. 

A very large bank will not eliminate your genset usage. If you don't match it with sufficient renewables (solar, wind), then a huge bank simply prolongs the interval between when you have to run the genset. The energy has to come from somewhere. We use about 225AH per day more or less. Our 960w array has been providing that and more, so we usually don't run the genset except to keep it functioning or while on passage. Note we rarely run AC, but we run the dishwasher at least daily, water maker every few days, scuba compressor now and then, washing machine twice a week, ice maker, Tv, etc. For our uses, 450ah is just perfect as it allows me to keep the batteries in a SOC range where they're happy. 


i ask Victron for his batteries a BMS for all batteries
There is my concern if this fails and it is not noticed all batteries damaged if each battery has a BMS is only one damaged.

You are right that it's a good idea to think about how you are going to fix things when they fail far from a dealer. I went through this thought exericise with our Victron setup. There is only one BMS, but if the BMS fails or there is a problem with the BMS circuit, charging and load are disconnected. I guess you can call that "fail safe" but the net effect is that all the batteries are protected. IMO, it's important to think about how you go forward if you're in the Tuamotus and that happens. If you have a backup, great. But for us, the batteries, should they not be damaged, will still work. If one of the 6 batteries are damaged, I take it and its pair out of the bank and move on. In the absence of a functioning BMS, my emergency plan is to bypass the BMS and monitor things manually (via bluetooth) until I can get a new BMS sent to me. 

For anyone considering any lithium system, you really should understand how the BMS works for your particular setup. If, for example, with a high cell voltage condition, it simply cuts off the batteries from the charging circuit suddenly, some chargers and I believe most alternators would be damaged. My system isn't perfect - for low cell voltage, for example, I don't like Victron's "cut off all loads" solution. What if I'm in the middle of a storm? I certainly don't want to lose autopilot and navigation.

I would not trust your supplier or your manufacturer to think through the unique situations we put ourselves in as far-off-the-grid cruisers. I have worked with both Victron high level techs and also a very well regarded dealer and they both gave me bad advice. 

For the one who use drop in, make sure they not only rated to run your bowthruster but also capable in real life, in some drop in the internat cables and BMS are not for high loads. can recommend Will Prowse on youtube very illustrative and easy to follow, he have teared a few drop in batteris apart and had a look what was inside, some of them you do not want to have onboard.

Agree with you there. I can tell you that the Victron Smart 150ah, run in serial pairs to get 24v and parallel to get to 450ah will easily run our Sleipner bow thruster with both the engine and chargers OFF. I spent a lot of time diagnosing a problem with our BT and did it all on battery. The draw is around 8000w I believe and we never saw less than 24.4v Our batteries supposedly can handle 2C continuous, so that means I should be able to draw 900amps continuously. Make sure your batteries can handle the rated draw, before you're away from the dock.

Victron sources Chinese cells but quality checks them and assembles them in a way that I trust. When they say the battery will deliver 2C, I believe it. I am sure if you are a capable engineer, you can do what Victron did for much less money, but I don't have that skill nor time, so I sadly just paid up!

I would caution against trusting that a drop in solution will work over the long term. Lithium is so different than lead acid in terms of charging profile, power delivery profile, how you keep them happy, the systems in place to keep them from failing, etc. Keep in mind that with one under voltage or over voltage event, you can kill a cell and therefore a battery, completely. While I am not overly concerned that Lifepo4 will explode like you cell phone battery and sink your boat, a battery system failure at the wrong time could have nearly the same effect if it's in the wrong place.


This is my biggest problem with Battle Born batteries. You cannot have sensible interfacing with other systems.
Lithium batteries like to be kept between 20 and 75% state of charge if you have extended charging facilities (i.e. shore power) . The voltage alone makes it very hard to determine the state of charge when a system is at use. Basically you need a Coulomb counter to see the SOC at any point in time. Ideally your BMS should provide you with this information and adjust the float voltage of any charging device in such way it can maintain a desired SOC after bulk charging. Given the different sources of charge you can have on a boat this is not so simple.

I believe Mastervolt makes staying within a SOC range quite easy, but MV is even more $$$ than Victron. On our system, the coulomb counter you speak of is our BMV-712 battery monitor. You do need to calibrate it monthly by charging to 100%, but otherwise it has a programmable relay that you can use to send a signal when battery SOC hits x and send another signal when SOC hits Y. You could easily use that relay output to tell your chargers to stop charging when SOC is at Y and to start charging again when SOC drops to X. All of the charging sources on my boat (solar, alternator, Quattro, Skylla) can be controlled by this. 

One last thing I would caution for anyone trying to figure out their own battery protection systems or even those using Victron is that you need to ensure that when the BMS sends a "stop charging" signal, the alternator actually stops charging and does so in a safe manner. It took me over a year to figure out how to get the Victron BMS to talk to our Mastervolt alternator charge controller in case the BMS senses a cell over-voltage condition. 

This is not something that is unlikely. In the last two years, I can recall at least 10 times when, approaching high SOC, we had an imbalance and one of the cells (out of 24 in our system) hit 3.8v and the BMS told the alternator to stop charging and the alternator stopped in a safe way to avoid damaging it. Aside: The BMS will balance the cells, but can't do it instantaneously and cannot do it with too high of charging current. If your alternator simply looked at the battery voltage or the bank voltage or the SOC, it wouldn't have seen the CELL overvoltage condition that battery would have been destroyed. The BMS must talk to the alternator.

Oh and make sure that the alternator has a temperature sensor and can taper output when it overheats because the max output duration placed on it by lithium is likely far longer than it ever saw with lead/agm/gel.

As a cautionary tale, I know of a former Amel owner who installed 1200AH (!!!!) of Victron lithium and they died within 2 years. He was quite wealthy, so I presume he paid someone a hefty sum and figured it would be done right. As with anything in a cruising boat, when something fails, the installer often shrugs his shoulders and the skipper deals with the consequences in the most inconvenient place and time. Even more so with a "newer" technology like lithium.


2007 A54 #69
SV Tengah

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