Temperature sensor for lead acid batteries

[GMB]

A long shot, but perhaps someone has encountered this:

I have found that I am missing a temperature sensor for a stack of lead acid batteries. All I know is that it was a 2-terminal device that looked like a match-box and must have sat amongst 12 lead-acid traction batteries.

It is not clear what it is used for. The reason for that is that the battery management for both charging and discharging is all handled by the same module even though the high power stuff is separate, and the device feeds into it.

My guess is that it may just be a thermistor but without an example it is a bit hard to know what value. Maybe it was a bimetal switch. With it missing things seem to work so by default it must be in the OK condition.

Perhaps someone has seen this kind of thing before and knows what the typical use might be. Any clues?

[Silicon]

The ideal charging voltage and trickle charging voltage varies with the temperature of the battery.

[wave solder]

On quite a few different solar charge controllers I have come across there has been a two core cable that extended down to where the batteries were situated and on the end of the wire was a small glass thermistor that was the same size and shape as a 1N4148 diode.

[Radio Wrangler]

You can even use a 2N4148. Si diodes give a temperature change of their "Vbe" of -2mV/C

David

[Ed_Dinning]

Hi Folks, I think there was also the Nat Semi LM35 ? temp sensor IC, like a 2 lead TO 92 transistor, this had a linear output curve

Ed

[GMB]

Yes, potentially many ways it could have been done (technology of 2007).
I now find I may be able to buy the original part - but at an eye-watering price.

It wasn't clear to me how critical float charge voltage would be as in tables that I have seen they often quote a range of values for each temperature, and these overlap considerably over the range you would get. I also see quite a bit of variability over what voltages (in detail) one would expect depending on manufacturer.

The complete absence of the sensor doesn't seem to stop things from working, but having said that all the batteries had previously mysteriously died (but not on my watch!)

[Bazz4CQJ]

Modern batteries are all too sensitive to over-charging. It's worrying to consider that a battery can be effectively destroyed if a simple diode or thermistor fails.
I normally charge my ~50Ah batteries on a homebrew charger with meters for both V and A. It developed a problem during the summer and, as a temporary measure, I bought a cheap "smart" charger. It was good for about three months then it cooked a battery (just kept charging at 14.8V). Fortunately, the battery was a very old one, and the episode has taught me a lesson.

B

[Roger Ramjet]

12V SLA batteries in burglar alarms are on a continuous float charge of 13.5 to 13.8 volts which was preset with the battery load disconnected.

Most batteries tended to last for the expected life of 5 x years, except ones where the alarm panel was located in warm locations i.e. industrial plant / boiler rooms, domestic airing cupboards etc.

Rog

[broadgage]

Lead acid batteries need a slightly higher charging voltage in cold conditions and slightly lower in hot conditions.
The difference is very small, and often an average or typical voltage will serve fairly well, unless extreme temperatures are likely.

If the battery is used gently with only slow charging and discharging, then local room temperature will often be a good enough approximation for the battery temperature.

If however the battery is fast charged and discharged rapidly, then losses can raise the battery temperature well above room temperature.

[G6Tanuki]

Originally Posted by broadgage

Lead acid batteries need a slightly higher charging voltage in cold conditions and slightly lower in hot conditions.
The difference is very small, and often an average or typical voltage will serve fairly well, unless extreme temperatures are likely.

If the battery is used gently with only slow charging and discharging, then local room temperature will often be a good enough approximation for the battery temperature.

If however the battery is fast charged and discharged rapidly, then losses can raise the battery temperature well above room temperature.

Yes, with lead-acid batteries designed for fast charging, quite a few of them have integrated temperature-sensors to detect the onset of unhealthy temperature-rises.

[duncanlowe]

I have a couple of CTEK chargers for 12v nominal lead acid batteries. One of them has a temperature sensor external to the charger, the other doesn't.

The SLA in our alarm panel lasted the best part of 20 years and then failed.

I did pick up a sensor from somewhere meant for charging batteries. It is meant to screw onto the battery post itself as the lead terminal post is a fair indication of the temperature of the plates inside.

[Bazz4CQJ]

So, an ideal lead-acid battery charger would sense by (i) voltage and (ii) temperature (or rate of temperature rise) and switch off on either reaching a limit. It does sound OTT, but the price of batteries is pretty eye-watering these days.

B

[G6Tanuki]

Yes. And if you can't do both sensings, at least do one.

Like you say, batteries are expensive and 'intelligent' chargers are relatively cheap.

Using an Un-intelligent charger that just keeps pushing out charge even though the poor battery is up to 15V+ and audibly bubbling seem a totally false-economy to me, as well as being deeply dangerous because of the risk of the battery venting or emitting Hydrogen.

[duncanlowe]

Originally Posted by Bazz4CQJ

So, an ideal lead-acid battery charger would sense by (i) voltage and (ii) temperature (or rate of temperature rise) and switch off on either reaching a limit. It does sound OTT, but the price of batteries is pretty eye-watering these days.

B

I think the most intelligent systems also include state of charge, by monitoring the current going in and out. That way, when it suits the charge voltage can be boosted above 15v safely to use spare energy, and dropped off when there is a requirement to save energy. I don't know if the current sensors mounted on the battery terminal also measure battery temperature (as described earlier) but I do know they know the ambient temperature.

The CTEK smart charges I have (as well as similar devices from Lidl and Aldi) have a complex charge sequence. It starts with a charge limited by current, as in the maximum the charger can deliver. Once the voltage has risen sufficiently, it moves to voltage regulated while monitoring the current, such that when the current drops to a particular level it reduces the voltage to the 'float' voltage where it will sit for quite a while.


The CTEK chargers also include a couple of pulse functions. If the battery is quite low when you start to charge it will pulse the voltage to help the start of charge. At the end of charge, you can also elect to pulse to recondition a tired battery, although I've not seen this do any good on a properly damaged battery.
Also worth remembering that a 'lead acid battery' has many variants. Gel electrolyte, absorbent glass mat, standard lead and lead calcium being common variations. And each has slightly different charging regimes.

EDIT: when I was a trainee, I worked in a department that had been asked to come up with a system for monitoring SoC on a 12v lead acid battery. They tried to do it based on terminal voltage. The closest they got was to turn on a fairly high power load, once the charge source was switched off to try to get a better idea. Even then the very small differnces in voltage were hard to measure and needed quite a precise voltage reference for the measurement. I remember said reference being called a widdler. In spite of the research it never worked well, and the move to actual current measurement as used today was the only apparent solution.

[Roger Ramjet]

The above well versed Duncan,

In the long gone good old days when all we had were "wet batteries", all one had to do was pop an hydrometer into the electrolyte to read the specific gravity versus temperature to accurately determine the state of charge.

Rog,

[broadgage]

The older types of large wet or flooded lead aid batteries were more forgiving of overcharging than are modern sealed types.

For service in the tropics it was recommended to use slightly weaker acid. S.G of 1.25 was often recommended for temperate climates and 1.2 for tropical climates.
If filing a battery with weaker acid it was important the battery had room for a generous volume of acid. Use of weaker acid in compact batteries carried the risk of limiting the capacity as the cell might "run out" of acid and cease to supply useful current, despite active materials remaining on the plates.
Use of full strength acid in hot conditions increases the risk of sulphation.
Use of weaker acid in cold climates increases the risk of a discharged battery freezing, and also increases the internal resistance of a partly discharged battery.

[GMB]

Update: I have had to obtain the correct part at huge expense.

The big surprise is that it didn't look as expected. In fact, it looks like nothing at all - just a metal tag as if for a cable termination. It seems they sense the temperature of the battery core through the actual connections. What I took to be the device itself was actually just an interface box - but this is the trouble investigating something that is missing!

It contains a tiny thermistor, worryingly puny compared to the chunky busbar terminal, so I will now carefully measure what it does in case it breaks again. I suspect this is a bog-standard thing but had no way to know if there is any kind of standardisation for these.

So if you ever find yourself taking apart hefty power connections on big batteries - just make sure that what may look like an extra washer isn't actually a critically important part!

[Jon_G4MDC]

Might it be possible to post a picture of the device please?

I have a long running battery charging / temperature issue and it would be good to know the enemy so to speak.

[SeanStevens]

Within most NiCad battery packs lies a little temperature sensitive resistor - I'm sure a small circuit could measure the resistance of one of those and cut off the charger . . . . ?

[GMB]

This is what it looks like: