Charging 8d and 4d wet batteries

[Brezzin]

On my new boat I'll have (3) 8d batteries and (2) 4d batteries (both are house)
Can I combine these wet cell batteries into one bank or should I split them. I know you shouldn't combine different type batteries (Gel or mat) but I'm not sure about same type different size.
Thanks

Dave

[camaraderie]

Dave...You CAN combine different size batteries of the same type without damaging them during charging....BUT.... over time there will be inefficiencies in the charging routine due to the different acceptance rates of the different sized batteries which will shorten their life somewhat. If you can afford them, I would recommend AGM's since they can take unrestricted amps during bulk charging but assuming you have a smart regulator for this, you will still be moving to acceptance and float rates too soon. The only way to really avoid this is to charge each house bank separately or simply move to 4 8D's rather than the combination. I also would not separate your house bank into 2 banks as this is also less efficient during charging...unless you have to stick with 2 different sizes due to space considerations. I've never heard of a new boat being spec'd out this way...is one set of batteries for a special purpose like a thruster or windlass?

[Dawkahab]

You can parallel them but it is not recommended. The 4Ds will charge more quickly and then potentially overcharge. You do not say what your charge rate is but it probably is less than 25% of capacity so it may be OK as you will not have sufficient charge capability to over charge, you may undercharge though. Regardless of that, it is not a good design.3 8Ds will be about 225 AH each total 675, 2 4Ds will be about 160 each 320 so you have about a thousand AH but not in a very usable package. You will be taking up about 8 square feet. Look at The Rolls modular 12 volts or the 6 CS25P the latter battery is 22x11.25 x 18.25 H That will take up 3.5 square feet and give you a premium battery package. They are 820 amp hrs but you won't miss the difference especially with all those cable connections and extra cells. 8ds and 4 4ds is 30 cells to maintain and lotsa cables, bad idea. If you have to have lower profile batteries and a thousand amp hrs you will be better off with a single bank of smaller 6 volts say in the 300-350 AH range. Without going into the details, I have been designing electrical systems for more years than I like to think about any more. Don't mix battery sizes, don't use a lot of cells and cables. There is never enough time or money to do it right but always plenty of both to do it over.

[Dawkahab]

Cam it is simply not true that AGMs will safely "accept unlimited charge rates" .Thermal runaway can cause excessive temperatures with potentially disastrous results. Read the paper by Jaworski and Hawkins

[Brezzin]

Well the boat comes with 3 8d's, the 4d's were simply to supplement the house bank. ( I didn't want a generator. On my current boat I don't think I've used the generator 30 hours in the last 3 years). I like the idea of the Rolls modular. I saw that at the Newport RI boat show. Going with 4 8d's makes much more sense as well. That should provide me with enough juice to be unplugged for a good long weekend with minimal engine running

Thanks, This is very helpful

[camaraderie]

Dawk...Lifeline batteries says about AGM's: "Faster recharge; no current limitations with voltage regulated recharging." Several other respected authorities say the same or at least in excess of the rated A/H capacity which will exceed anything available on a boat!

After reading your post to Brezzin which I entirely agreed with, I was motivated to read the article you cited to see if I was wrong but I'm not. I think you misunderstood the results and the conditions of the experiment. First let me cite exactly what I said:
"I...recommend AGM's since they can take unrestricted amps during bulk charging "

Now here are the experimental conditions from their paper:
"All experiments were performed on fully charged
batteries on constant voltage float placed in an
environmental chamber which allowed ambient
temperature control to better than 0.2°C."

So... I am talking about bulk charging which can be done at "unlimited amps" at no more than 14.3 Volts and you are talking about FLOAT charging which on boats is done at about 13.8V or less and an amp or two of current.

Now thermal runaway might still be of concern to us all using AGM's on our boats so let's look at what was done to induce thermal runaway. AMPS were not varied VOLTAGE was!! And the voltage was varied (on a 3 cell battery) to the equivilent of between 14.5 volts which did not produce thermal runaway to 15.5V which did. Remember that float voltage should be 13.8 or less.
Thus we conclude that AGM's in boats running a normal under 14V float charge are perfectly safe* from thermal overload as well as being able to accept practically unlimited current during bulk charge.

Am I reading any of this wrong??... or did you simply look at the runaway current graphs and not realize they were being induced by excess charging voltage which raised the internal temperature of the battery?
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*note that in the event of a shorted cell or two (i.e. a defective battery) thermal overload can be encountered at normal float voltages so batteries do need to be monitored.