Battery heat

[nzfisher]

This a general question about batteries in an R25. I recently added 2 batteries to the house circuit's original 2, making it more like the R29's 4 house batteries. I added 2 that are the same as original which are in pretty good shape (charged voltage just over 13 volts.). In there new location the terminals are more exposed to an inadvertent short across the terminals than the existing batteries in their own special compartment. My questions are-

Can I fabricated a polymer cover for the new batteries without needing to worry about heat creation and need for dissipating heat from the 2 new batteries?

Am I going to run into problems because the new batteries are being charged at the same time as the old house batteries? By trouble I mead serious issues like potential for fire. If so, I am willing to replace the other AGM house batteries as necessary or switch the new ones so they are only being charged when I need them.

Lastly, are all of the battery circuits charged at the same time if all the switching is on (red main switches). To me this would mean that the charging might be based on the weakest voltage sensed from the entire group of batteries.

This is one area of the boat where my knowledge is more limited than others and I thank those more knowledgeable for their assistances in my questions.

 

[Levitation]

In general:
Yes, you must insulate the exposed terminals on the new battery set. Drop a wrench across the terminals and the ball of fire that ensues will be something to behold - that is if you could still see at that moment.
Heat dissipation is a minor issue and not enough to stop you from insulating the terminals. Just do it. Now insulation could be as simple as painting the exposed clamps and posts with Liquid Tape 😉
I like having a switch to isolate the second battery set and I did install one on mine - though 99 & 44/100ths % of the time they are not isolated (shrug).
I do not anticipate any problems with your smart charger. It has three separate charge banks (engine/thruster/house). The fact that you have doubled the size of the house set merely means it takes a bit longer to recharge the house that it did prior to 4 batteries. The other two banks will charge and go into float mode same as before whilst the house set continues to merrily bubble away towards being charged.
Now, this last statement is based on Ranger having continued the battery/charger setup on the Cutwater the same as on Tugs.
If not then I am wrong - of course that would be the first time in forever :mrgreen:

 

[captstu]

Flooded batteries don't heat so much if the charging is at a moderate rate.

For any given rate of charging, four batteries will split the heat on each battery "eavenly", allowing twice the heat dissipation area the original two batteries provided.

For example, if the original charging maximum rate was - say - 50 amps - split between two batteries, you had to dissapate the heating load of 25 amps each. Now, 25 amps x 12 (or so) volts is a maximum of about 300 watts into each battery. Most of that power goes into the charging process and is stored in the battery. If the batteries are - say - 90% efficient on charging, 30 watts is lost as heat - warming the battery until the waste energy is dissipated as heat.

Now, if you add two more batteries, instead of 30 watts in each battery, only 15 watts must be dissipated or used to heat the battery.

Next time you start charging, feel the batteries after an hour or so of running. Typically they will only be slightly warm - four batteries will warm less than two.

This isn't the problem we need to worry about.

A 400 Ah bank such as your 4 battery bank is (approximately), can provide a short circuit current well in excess of 1000 Amps. Should one of the 6 cells in each battery develop an internal short, that battery may [produce only 10 volts rather than the rated 12 volts. The remaining 3 batteries will try to "charge" the failed battery, dumping 200 Amps or so into the shorted cell until it overheats to the point of boiling and eventually melting. In the meantime, gas will be expelled from the damaged battery containing hydrogen until something series happens, stopping the reaction.

This happened to me with a set of 8 series/parallel golf cart batteries on a previous boat. The battery box cover charred, smoke and gas was everywhere, but fortunately there was no explosion. Myt battery bank was fused as a bank, but unfortunately neither the designer or I considered the internal shorting problem, called RUN AWAY.

I was able to cut the wires to the failed battery, grab the mess with cooking cloves and drop it overboard, preventing a much more serious fire.

After reading about run away batteries, I suggest you separately fuse the new bank to the mains rather than just connecting the four batteries together into a single larger battery. That way, if one internal cell shorts, you will have some protection - and two remaining batteries to get home.

Wikipedia has a better explanation of the process at https://en.wikipedia.org/wiki/Thermal_runaway

Runaway is a serious problem with Lithium batteries, my failure was on AGM batteries, but it can happen to any battery if a large enough bank is hooked together.

 

[knotflying]

Regarding protection of the terminals. You can easily purchase rubber covers that go over the terminals. Most auto pats stores carry different varieties.

 

[SGIDAVE]

This a general question about batteries in an R25.

Lastly, are all of the battery circuits charged at the same time if all the switching is on (red main switches). To me this would mean that the charging might be based on the weakest voltage sensed from the entire group of batteries.

Hello NZ,

Most of your questions have been addressed except this one...

It's important to realize, that since our boats have ONE alternator as their underway charging source, the boat's batteries will ultimately be tied together electrically such that they act like ONE large bank for charging purposes. At least when they are being charged whilst underway.

Compare now the charge process when the boat is connected to shore power and the AC-powered charger is doing its thing. Often, the same Automatic Charging Relays (ACRs) that served to combine the batteries whilst underway so that all could receive charge from the main engine's alternator, now do the same thing and end up combining all, or most, of the batteries. Look at your own set up. Even though you have a 3 bank charger, if the ACRs function to "combine" the batteries, you are really getting single bank charging.

It boils down to the locations of the battery's ON/OFF switches relative to the ACRs. If the ON/OFF battery switches are located between the batteries and the ACR, then if you turn off the battery switches you will actually get individual bank charging. If the ACR is between the batteries and the ON/OFF switches, then the ACRs will combine the batteries during the charge process (as they are designed to do) and you end up with single bank charging.

/dave