Jump to content

Trojans behaving Badly


dmr
 Share

Featured Posts

no its me telling lies help.gif i put the midnite controller in at the same time which has a widget that sticks to the battery. in my defense i removed a widget from the tracer that was in, and repositioned the widget from the whispergen toooo many widgets

Edited by peterboat
Link to comment
Share on other sites

However I suggest it is a bit like Peukert, rather misleading for boat applications. When the batteries are cold, there is the same amount of chemicals to react and thus the same number of electrons to be migrated, the reaction just happens more slowly. Ultimately there is the same capacity available if you discharge slowly. But of course in terms of a battery's spec, capacity at C20 say, a cold battery will reach its terminating voltage sooner than a warm one and thus the effective capacity at a constant C20 is reduced. However if you took that ostensibly flat battery and either warmed it, or left it to recover and then discharged slowly, you could recover the remaining charge. The charge is not lost in cold weather, it's just hibernating like all sensible things!

 

 

That's a very interesting and plausibly hypothesis.

As the forums No1 tester you could do an experiment for us.

You know that your batteries are fully charged most of the time and you know their capacity, so on a cold day take a measured number of amp hours out of them and then measure the rested voltage and/or specific gravity????

But what then? I assume both will show a lower than expected state of charge due to the "lower capacity". You then somehow need to wait a long time and/or warm them up again and see if the lost capacity comes back....all a bit tedious and intrusive.

 

................Dave

Link to comment
Share on other sites

No, it doesn't need one. The rested voltage vs SoC graph only has a very slight temperature co-efficient, no enough to bother with. Your solar controller needs one because the optimum charging voltage changes a lot with temperature. I can't immediately think why a nasa bm2 would need one.

 

Edit: according to the nasa bm2 manual I've just looked at, there is no temperature probe.

 

You are correct, I didn't know this, had not really thought about it too much but had naively assumed the rested voltage temp coefficient was the same as the charge voltage coefficient.

www tells me 1.3mV per degreeC (12volt battery) which is small. This would cause the smartgage to read 97.5% on a fully charged battery which is a very small error.

So it looks like we need the higher charge voltage on cold batteries because its harder to make the chemical reaction happen, and not to overcome the higher internal voltage.

 

...................Dave

Link to comment
Share on other sites

You are correct, I didn't know this, had not really thought about it too much but had naively assumed the rested voltage temp coefficient was the same as the charge voltage coefficient.

www tells me 1.3mV per degreeC (12volt battery) which is small. This would cause the smartgage to read 97.5% on a fully charged battery which is a very small error.

So it looks like we need the higher charge voltage on cold batteries because its harder to make the chemical reaction happen, and not to overcome the higher internal voltage.

 

...................Dave

Plus of course if you link it to charge voltage, that means the terminal voltage on discharge would increase significantly at colder temperatures and I don't think anyone is suggesting that!

Link to comment
Share on other sites

 

That's a very interesting and plausibly hypothesis.

As the forums No1 tester you could do an experiment for us.

You know that your batteries are fully charged most of the time and you know their capacity, so on a cold day take a measured number of amp hours out of them and then measure the rested voltage and/or specific gravity????

But what then? I assume both will show a lower than expected state of charge due to the "lower capacity". You then somehow need to wait a long time and/or warm them up again and see if the lost capacity comes back....all a bit tedious and intrusive.

 

................Dave

I would suggest there will be a loss of energy, as the energy used within the battery to power the chemical reaction etc will be more than at say 27C. Thus any Ahs recover on warmup or after time waiting will be at a lower voltage. Difficult to generalise how big a loss of energy, depends on the battery and the temperature.

Link to comment
Share on other sites

That's a very interesting and plausibly hypothesis.

As the forums No1 tester you could do an experiment for us.

You know that your batteries are fully charged most of the time and you know their capacity, so on a cold day take a measured number of amp hours out of them and then measure the rested voltage and/or specific gravity????

But what then? I assume both will show a lower than expected state of charge due to the "lower capacity". You then somehow need to wait a long time and/or warm them up again and see if the lost capacity comes back....all a bit tedious and intrusive.

 

................Dave

No, I would say that with cold batteries, taking X amphours out results in the same specific gravity (having compensated for temperature) as you would have got taking the same charge out of the battery if it were warm. If not, what happened to the electrons migrated when the chemicals reacted so as to reduce the sg? I come back to x mass of chemicals moves y electrons where y =kx (can't be bothered to work out k!). Of course with a cold battery, there will be more voltage droop (slower reaction) and hence less energy, but of course we have to be careful to distinguish between charge and energy.

 

So the voltage during discharge of the cold battery will be less, but once the batteries are rested I don't see why the rested voltage should be any different from what it would have been with warm batteries, barring the very slight co-efficient you mentioned, although no doubt it would have to be rested for longer to get the voltage to stabilise.

Link to comment
Share on other sites

No, I would say that with cold batteries, taking X amphours out results in the same specific gravity (having compensated for temperature) as you would have got taking the same charge out of the battery if it were warm. If not, what happened to the electrons migrated when the chemicals reacted so as to reduce the sg? I come back to x mass of chemicals moves y electrons where y =kx (can't be bothered to work out k!). Of course with a cold battery, there will be more voltage droop (slower reaction) and hence less energy, but of course we have to be careful to distinguish between charge and energy.

 

So the voltage during discharge of the cold battery will be less, but once the batteries are rested I don't see why the rested voltage should be any different from what it would have been with warm batteries, barring the very slight co-efficient you mentioned, although no doubt it would have to be rested for longer to get the voltage to stabilise.

 

Thinking outline rather than arguing with you (it is a discussion forum after all).......

I'm not so sure. Rested voltage and SG are the two proper ways to measure SoC and normally go hand in hand, so we might not expect the two to diverge. You are suggesting that after a cold discharge the SG will be correct but the voltage will be low, but if left alone will recover?.

We are hypothesising that this is like Peukart and is a surface effect mechanism?, the plates over discharge at the surface because the reaction is slow, and then recover later as the deeper parts catch up. Once we start thinking this way it all gets even more confusing because we probably need to stir up the acid to get a good reading of "bulk" SG, not so good with your optical device.

I can split of two of my batteries so if we have another cold snap at a convenient time I might be able to investigate this a bit....discharge the batteries then split off the two and watch to see if the rested voltage starts to increase?

 

Trouble is, although I have seen reduced capacity before during cold weather, I have never seen anything this dramatic before so it looks like spending a week in a house right on the beach in Cornwall is the critical factor. Would the forum have a whip-around to finance another holiday for me????

 

................Dave

Link to comment
Share on other sites

Thinking outline rather than arguing with you (it is a discussion forum after all).......

I'm not so sure. Rested voltage and SG are the two proper ways to measure SoC and normally go hand in hand, so we might not expect the two to diverge. You are suggesting that after a cold discharge the SG will be correct but the voltage will be low, but if left alone will recover?.

We are hypothesising that this is like Peukart and is a surface effect mechanism?, the plates over discharge at the surface because the reaction is slow, and then recover later as the deeper parts catch up. Once we start thinking this way it all gets even more confusing because we probably need to stir up the acid to get a good reading of "bulk" SG, not so good with your optical device.

I can split of two of my batteries so if we have another cold snap at a convenient time I might be able to investigate this a bit....discharge the batteries then split off the two and watch to see if the rested voltage starts to increase?

 

................Dave

I'm confused, you say "not so sure" but then seem to go on to agree with me.

 

Anyway regarding the refractometer I've only previously used it on a fully charged / equalising battery where the electrolyte is presumably well mixed by gassing. As I discovered when I found the sg unreasonably high the other day at low SoC, mixing the electrolyte seems to be necessary. I do have a "turkey baster" that I got for adding water, so that could be used, although to some extent that then negates the point of the refractometer over a hydrometer.

 

As with all such tests, eliminating experimental error would be tricky due to the relatively large impact of the variables vs the relatively small expected result, not the least of which is that I'm coming to the conclusion that a better mental model of a battery is as a living thing rather than as a bucket.

Link to comment
Share on other sites

Well... the effect of the lowered capacity is caused by a higher internal resistance. This explains why the standing voltage remains the same but lowers dramatically on discharge. Interestingly, the high internal resistance will actually assist the battery by warming it up once you start to discharge it.

 

Edit to add a link to Battery University's take on low temps: http://batteryuniversity.com/learn/article/discharging_at_high_and_low_temperatures

 

... I'm coming to the conclusion that a better mental model of a battery is as a living thing rather than as a bucket.

... often in a bad mood to boot :) Edited by WotEver
Link to comment
Share on other sites

Well... the effect of the lowered capacity is caused by a higher internal resistance. This explains why the standing voltage remains the same but lowers dramatically on discharge. Interestingly, the high internal resistance will actually assist the battery by warming it up once you start to discharge it.

Edit to add a link to Battery University's take on low temps: http://batteryuniversity.com/learn/article/discharging_at_high_and_low_temperatures

... often in a bad mood to boot :)

Perhaps we need to determine the gender of batteries? Even that doc is likening them to human beings!

 

Anyway, the doc just says the capacity is reduced which of course it is, since capacity is defined as discharging it at the specified rate until it reaches a specified voltage, and the cold battery will do that sooner. But at that point, more reactive chemicals remain than if the battery had been warm and discharged for longer.

Edited by nicknorman
Link to comment
Share on other sites

If you want a permanent and reliable source of electricity, go live in a dwelling that has a connection to the national grid..

 

If you want to live on a boat, consider oil lamps. candles. Using a kettle on the stove.

 

Or consider your precious batteries for what they are. A consumable..

Link to comment
Share on other sites

If you want a permanent and reliable source of electricity, go live in a dwelling that has a connection to the national grid..

 

If you want to live on a boat, consider oil lamps. candles. Using a kettle on the stove.

 

Or consider your precious batteries for what they are. A consumable..

But I want multicoloured LED lighting that "dances" to the music, recording HD satellite box and tv, a Nespresso coffee machine, electric kettle, toaster, bread machine, dyson, automatic washing machine, tumble drier, electric blanket, keyfob remote control of some functions, control and telemetry via mobile phone.

 

Oh that's lucky, it seems I can have all that and have batteries that last several years. Phew!

Link to comment
Share on other sites

 

Trouble is, although I have seen reduced capacity before during cold weather, I have never seen anything this dramatic before so it looks like spending a week in a house right on the beach in Cornwall is the critical factor. Would the forum have a whip-around to finance another holiday for me????

 

................Dave

 

 

An even more interesting experiment would be to see if your batteries display the same behaviour if it's me having the holiday in your Cornwall beach house :D

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.