Alfabiker

Dr Bob: what did you conclude about your alternator temperature measurements? As I'm thinking of a hybrid system (see your other thread about the "affordable way to add lithium batteries" etc.) I intend to do some measurements of the alternator temperature when charging the existing LA bank first, to get an idea of the normal working temperature. Obviously charging an extra 100-120Ah Li bank in parallel with the existing LAs (345Ah) is likely to work the alternator a lot harder. In fact I spoke to Beta about this and they said that at least one boater had experienced an alternator that got hot enough to set off a fire(?) alarm when they installed a bank of lithium batteries, but I don't know how large a bank that was, so they were somewhat negative about the whole idea. On the other hand, I was somewhat encouraged to see the comments from Boater Sam to the effect that the Iskra altenator should be quite happy at fairly high temperatures - they must be self-regulating to some extent when you consider the large variation in sizes of boat battery banks and the low SOC that some people run down to - but in any case my idea was to add a digital thermostat device (of a kind that's pretty cheap on Amazon and can go up to 120 deg C) to open a suitable relay and just disconnect the LI bank if the temperature should rise above the desired limit.

sorry I meant "overheat the alternator" of course!

Just coming back to my original post for a moment, I don't have a separate alternator controller, but my alternator (70A on a Beta 38) does produce 14.4 volts anyway, I wasn't really concerned that I wouldn't manage to charge an LI battery sufficiently. It was rather that the low resistance of an Li battery might cause the charging current to rise too much and overheat the controller (I assume that the resistance does rise as charge increases though?) But again, there don't seem to be any caveats about this in relation to "drop-in" LI batteries so what is the story here?

I was beginning to wonder about this, as you do read different suggestions everywhere you look! My alternator which is quite new, puts out 14.4-14.5 volts maximum, and we generally cruise for only about 4-5 hours a day. Also, I always check the voltage of my battery bank first thing in the morning when it is lowest and off load (as far as possible) to make sure it is above 12.2v. It seems to me that if I can find a reasonably reliable BMS to take care of possible extreme circumstances - such as happened a couple of years ago when our old alternator went rogue and cooked a set of AGM batteries(!) - there shouldn't be too much of a problem. As mentioned above I would in any case put in a manual disconnect as I don't want the Li batteries to be floated by the solar when we are away from the boat (even though float is set to 13.2v to treat the SLA's gently). Of course there are also quite a few "drop-in" Li 12v batteries for boats etc. which claim the same sort of figures as nicknorman has quoted, but one does have to maintain a healthy level of scepticism about their built-in BMS I suspect!

Dr Bob: I am very interested in your idea of the simple hybrid system as I can see a lot of advantages in it. I presently have 3 Exide 115ah SLA's in the domestic bank of my boat (and 300w of solar) and I could do with a bit more capacity, so I was thinking of adding a LiFePO of say 120Ah in parallel, hopefully with some suitable controls, but I have a couple of basic concerns: (1) You mention that you can run the combined bank down to 12.0 volts, but this seems a bit low for the recommended working range of the LiFePO batteries - some people even talk about disconnecting them at 12.7-12,8v (I try not to let my SLA bank run down to less than 12,2v off load anyway)? and (2) my boat engine has a 70A alternator for the leisure batteries (as well a separate smaller one for the starter battery) which charges the existing bank very well, but do I run the risk of overloading it with the addition of the one extra Li battery or should I get away with that do you think? Thanks for your thoughts in advance!