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Carlos G

Wet cell battery temperature

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Just had a potentially dangerous incident with the sailboat house bank. It has 3 gp27 batteries fed by the normal alternator setup and by a 130 watt solar panel with a controller. Bank is one year old. I noticed the battery compartment hot and checked with the IR thermometer. The battery in the middle was at 140 F, the other 2 at 90 F. Took it out and it stayed warm for many hours. Apparently an internal short. The question is: how to monitor all batteries in the two banks and how to disconnect or isolate a bad battery while the boat is unattended. Thanks in advance for ideas.

7 Replies

  • Additional comment to original post: will fuses between batteries help to avoid or minimize a catastrofic failure? What size fuse if bank is used ocassionally to start Yanmar 44 hp? House load is not more than 15 amps per hour. Carlos.

  • Carlos, you should have a good fuse on the battery bank but I don't think that fuses between batteries will do much good. I don't know if my "catastropic failure" of one 8D generated any heat or power surge; what was catastrophic was that it suddenly lost all its power (and then pulled down the other 8D) much faster than it should have.

    At any rate it is possible to measure each battery's temperature but it may take some sophisticated gear to do it well. I'm thinking a Maretron TMP150 and also a Maretron display so can label the temps well and also give high values alert and alarm status.

    It seems to me that it's also possible to have one ambient temp sensor in a battery compartment. If any one battery gets unusually hot the whole compartment should get unusually hot.

  • Thanks. By catastrophic I mean possible fire. Idea about fuse between batteries is thinking there may be a high current flow between batteries. Trying to find a mechanism to stop or minimize the problem when nobody is in boat.

  • Sometimes the best insurance for some of these unusual problems, is the insurance you purchase from your broker.

  • Quote: "Trying to find a mechanism to stop or minimize the problem when nobody is in boat."

    Unfortunately with batteries in parallel fuses almost always won't trip them when a cell shorts.

    In a perfect world all marine battery charge sources would have the ability to wire in as many temp sensors as you have batteries and any one of the sensors could then shut the charger down.

    Sadly we don't live in a perfect world and the marine charge equipment market has failed us on the safety front. One could easily argue that the ABYC has let us down on this front too. Seriously why is one sensor standard practice and "acceptable" on a boat with ten batteries..?? The simple answer is that it should not be acceptable.

    The technology to do this is easy, industry gold standards such as the LM335 temp sensors are 96¢ each. Added a bit of wire and a terminal with some epoxy and charge $40.00 each and they are actually making money in a big way. It would at least be nice even if it was an add on option but as of yet not one single charger company I know of offers multiple battery temp sensors. The Balmar MC-614 can monitor two batteries, a step in the right direction, but that is still not enough on boats with multiple battery banks..

    As for what you can do?

    #1 Conduct yearly Ah capacity tests. This is as easy as applying a load of;

    Ah Capacity ÷ 20 = Applied Discharge Load

    A 100Ah battery would get a 5A load until the battery voltage hits 10.5V. Try to rig a system that lets you adjust the load as the voltage decays to keep it steady. A bank of light bulbs or resistor works or even a PWM 12V dimmer can work to keep the load as close to C÷20 as possible. Recharge immediately after hitting 10.5V.

    Start a count down timer (there are free apps on your phone for this) when you turn the load on and note the time the battery ran for and remained above 10.5V. For example a 100Ah battery that ran for 16 hours at 5A is delivering 80% of its 20 hour Ah capacity rating.

    Conduct the test with the battery temp between about 74F and 80F or the capacity will be off a bit. Any battery falling below 80% of its stated capacity is getting into the "time to think about new batteries" zone.. For coastal cruising as low as 70% with AGM or GEL can be fairly safe but if going off shore you really don't want them to drop much below 80% of rated capacity. Below 80% is where I start to see trouble occuring.

    Impedance testers (Midtronics, SPX, Argus etc.) really don't tell us much about state of health as a house bank and really nothing at all about Ah capacity, but if you start with a baseline, when new, they can tell you a little bit of a story/history.

    #2 When leaving a vessel unatended it's best to use the absolute smallest charger necessary to maintain the bank. In my opinion one of the poorest practices I see at docks is using 100A - 150A + inverter chargers as your maintenance charger. Having a 150A potential in order to supply mA of charge current when your not there, not my bag of tea...

    Why is this a poor practice?

    With a 5A charger if you have a shorted battery, that may now be a 10V battery, that battery will begin sucking charge from the 12V batteries but the differential voltage is still around 2V or less. When you stack a small 5A charger on top if this, it does not create a real huge difference. Add a 150A charger on top of four other 12V batteries feeding the shorted one and you've got real issues. That 1 temp sensor is almost never going to be on the shorted battery. This is the failing of the charger industry I talk about.

    #3 Internal shorts most often start out small and present themselves in faster than usual voltage decay when not charging. When charging they present themselves in a manner that the charge current never drops as low as it once did. For example with no loads on, with a typical float voltage, your bank may have normally been accepting 0.5A of charge current and now it never drops below 9A.. This is a red flag.. If you pay close attention to your ammeter and battery voltage, using an accurate voltmeter that reads to the hundredths (12.72V) not just tenths (12.7V) then you can learn to spot any irregularities in the bank voltage and full charge amperage performance.

    Large chargers left unattended are simply going to remain a dangerous practice until charger makers begin to provide us with the ability to sense the temp of every battery on board...

    You as a human are still the best defense against battery failures. I rarely see them fail rapidly and more often than not see them fail starting out as a small leak/short the slowly gets worse...

    Course if you are on-board your Model 1 Schnoz should alert you to a shorted battery.

    Shorted Trojan 154F

  • Compass: Thanks! Excellent clarification and suggestions. Will adjust my maintenance routine. Will a temp sensor work if connected to a metal strip attached to the 3 batteries? Will the controller stop feeding the batteries? I hope we can get manufacturers involved. This type of problem seems to be a dangerous and silent enemy.

  • I totally agree with Compass. I just had an incident on my boat. One out of two starter batteries shorted and got very hot.(194F)

    None of the other 9 batteries in the box was hotter than 86F. The ambient temp. In the box was 97F.

    One temp.sensor strategy simply does not work.

    Luckily I was on the boat and could take care of the problem. I think Compass suggestion of a smaller charger is good if you want to leave the boat on shore power. Another possibility is to put the charger on a timer and have it go on once a week for some hours. If the time choosen is short I do not think that any serious heat will develop.