DIY Lithium

Petes

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Screenshot_2021-04-08 Malcolm - VRM Portal.png


This shows the BMS doing its stuff today and you can clearly see the 1hr absorption before it dropped to float. the Pi and mifi take a standing 0.4A. I t didnt start until lunchtime due to the temperature dropped to 3C so no chrage to the battery until it warmed up.
Think it shows it very well.
 

1 Cup

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Looks like the heat pads alone are no choice touching cells , hot spots!. What if you heat encloser / box If you need to charge every day at sub zero temps.
Im thinking of something like under floor heating pipe that also go around water tank, using the bobil tank system.
its feed. at 60 + degrees for shower but will adjust. To lower temps just have

Theres a small system on youtube foe boats for batteries useing both lead and lion 4, in line. but I can't remember his daily use. I think no inverter.
 

SquirrellCook

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I think the only way to uniformly warm them would be to circulate warm air around them. This would upset compressing them. Or to use aluminium plates between and on the ends of them with some Raychem wire potted in each plate. Better still don’t let them get cold.
 

Derekoak

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I dont think there is anything wrong with heat pads, if the maximum temperature stays below say 30 C any odd hotspot to 40 C would dissipate quickly. I am not at all sure that my first stab at the maths was accurate. I have been trying to research the maths of heat transmission around the aluminium cell walls and then in to the core of the cell. Heat transmission in series mimics electrical transmission through resistances in series.
@ I cup the paper I linked measured heat emitted from the terminals. They are after all connected by conductors to the anode and cathode in the cell. The resistance was about 8 times higher than through the big face.
 

Brockley

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As previously posted, this works fine, the recent cold snap has proved it, no worries about the heat pad getting too hot, the thermostat looks after it. The Bluetooth lets you know exactly what the battery is doing heat wise.
I have made an aluminum plate folded to come slightly up the sides and ends of the battery. Alluminium tape securing foil backed polystyrene all round the battery and to the plate. The little illuminated box behind the battery is a thermostat powered by the battery controlling a rubber heat mat which is fitted directly under the battery on the alluminium plate. The thermostat kicks in when the temperature drops to 4 degrees and switches off when the temperature reaches 15 degrees. The temperature probe from the thermostat is directly under the battery inside the plate, so it’s not running for too long before reaching cut off temperature. The thermostats cost about £2.50 and the heating pad was £15 and uses 100watts while operating. It’s only operated a couple of times over the last couple of weeks and no real drain at all.View attachment 93705
 
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SquirrellCook

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I dont think there is anything wrong with heat pads, if the maximum temperature stays below say 30 C any odd hotspot to 40 C would dissipate quickly. I am not at all sure that my first stab at the maths was accurate. I have been trying to research the maths of heat transmission around the aluminium cell walls and then in to the core of the cell. Heat transmission in series mimics electrical transmission through resistances in series.
@ I cup the paper I linked measured heat emitted from the terminals. They are after all connected by conductors to the anode and cathode in the cell. The resistance was about 8 times higher than through the big face.
I remember similar looking equations when I was doing 1st. year O level physics and they look about right to me, but I don't thick they would apply to a complex structure. To be honest anyone who has worked with heat dissipation and propagation would be able to make some very good guesses. These days I avoid maths as there is too much chance of making an error. Safest bet would be to make a simple test rig.
 

wildebus

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I remember similar looking equations when I was doing 1st. year O level physics and they look about right to me, but I don't thick they would apply to a complex structure. To be honest anyone who has worked with heat dissipation and propagation would be able to make some very good guesses. These days I avoid maths as there is too much chance of making an error. Safest bet would be to make a simple test rig.
Where possible, real-world testing is always a good idea (y) So many factors and unknowns to consider when just doing the sums (and how do you model an unknown?)
 

Derekoak

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Yes real world testing is the way. however Dan was the only u tube video showing a real test. Phil and harry have probably missed the cold to test properly this year. Unless they put their batteries in a freezer! Brockley has experience perhaps that is best. I am last minute deciding the parameters for mats from China, perhaps with significant shipping so I have a reason to try to understand now.
 

Derekoak

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As previously posted, this works fine, the recent cold snap has proved it, no worries about the heat pad getting too hot, the thermostat looks after it. The Bluetooth lets you know exactly what the battery is doing heat wise.
Thanks for reshowing me that. Clearly it can be very simple. As the sensor is right by the mat it cannot overheat. As you keep the battery always warm it will not be really cold in parts.
The only difference to what I was imagining is I was hoping to not keep the batteries ( in my case underslung remember) warm all night when there is no charging going on but let them drop with ambient. I think even with an inch of foam insulation that might take significant charge to keep them warm all night in negative C? Then when a charge (sun or alternator) is possible use that power to heat the battery relatively quickly until it is safe to charge the battery. Dan I think had a similar plan with a manual switch. I also think @Phil had a simple circuit to do this.
Do you have a link to the heat pad you have used? Did it take long to ship?
 
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Nabsim

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Inside the van works fine for me, I did toy with heat pads but I could run a pipe from the heating if I needed it. I have had them in two winters now with no problems. Well I have had one in for two winters, it may have been early spring when I fitted the second one
 

Derekoak

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I remember similar looking equations when I was doing 1st. year O level physics and they look about right to me, but I don't thick they would apply to a complex structure. To be honest anyone who has worked with heat dissipation and propagation would be able to make some very good guesses. These days I avoid maths as there is too much chance of making an error. Safest bet would be to make a simple test rig.
Thanks for looking, yes the structure is not simple. It needs more sophisticated maths.
 

wildebus

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Yes real world testing is the way. however Dan was the only u tube video showing a real test. Phil and harry have probably missed the cold to test properly this year. Unless they put their batteries in a freezer! Brockley has experience perhaps that is best. I am last minute deciding the parameters for mats from China, perhaps with significant shipping so I have a reason to try to understand now.
They could move to scotland? had a sheet of ice 1/2" thick in a bucket of water two days ago!
 

wildebus

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They could move to scotland? had a sheet of ice 1/2" thick in a bucket of water two days ago!
Just been out to pick up my Camper post-MOT .... gritter doing its thing coming the other way! (Oh well. Guess it is not the 12th July yet)
 

Brockley

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Thanks for reshowing me that. Clearly it can be very simple. As the sensor is right by the mat it cannot overheat. As you keep the battery always warm it will not be really cold in parts.
The only difference to what I was imagining is I was hoping to not keep the batteries ( in my case underslung remember) warm all night when there is no charging going on but let them drop with ambient. I think even with an inch of foam insulation that might take significant charge to keep them warm all night in negative C? Then when a charge (sun or alternator) is possible use that power to heat the battery relatively quickly until it is safe to charge the battery. Dan I think had a similar plan with a manual switch. I also think @Phil had a simple circuit to do this.
Do you have a link to the heat pad you have used? Did it take long to ship?

It didn’t take long to arrive as it was a U.K. seller, the pad fairly closely matches the footprint of my battery but smaller enough to put some packing strips in either side of the power cable and also just after the far end of the pad so that there is a slight cavity under the battery enough to insert the heat sensor probe, maintain an even air gap under the battery and not allow the battery to put any pressure on the heat pad.

My battery is totally insulated with the stuff you stick on walls behind radiators (silverside out). I used Aluminium tape to seal everything to the moulded Aluminium belly pan.

For inside the van this works very well, I get what you’re saying about your underslung situation, but surely that just requires better insulation (the two inch thick foam box my battery was delivered in springs to mind, if there’s enough space)?

As for wasting electricity on warming a battery overnight, I’m with you entirely, but if you can insulate adequately, a gentle clicking on and off 100watt heat pad uses 8.333 amps when on. With reasonable chargers, b to b, hook up and solar that draw is a drop in the ocean. Decision decisions, ain’t it fun?
 

Brockley

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This is the thermostat I chose, this particular one is dearer but local. I bought 3 of them direct from China because they are much cheaper, very accurate and way easier to set than the two other common types (both of which I’ve used with much frustration on other projects).

One tip - power in cables are easy - red positive, black negative. On load out you need to think Chines, they say yellow is positive and black is negative, I doubt it would matter which way round to the heat pad but that’s what they told me and that’s what I did. Works just fine.
 

Derekoak

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This is the thermostat I chose, this particular one is dearer but local. I bought 3 of them direct from China because they are much cheaper, very accurate and way easier to set than the two other common types (both of which I’ve used with much frustration on other projects).

One tip - power in cables are easy - red positive, black negative. On load out you need to think Chines, they say yellow is positive and black is negative, I doubt it would matter which way round to the heat pad but that’s what they told me and that’s what I did. Works just fine.
I really like your system simplicity. However What happens if you leave your camper in bad weather? Now the batteries keep themselves warm until they hit the Bms low voltage cut off. Now they will not charge or discharge and get cold. When the alternator runs or the sun comes out, what happens? Can they use charging current to warm themselves?
 
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Derekoak

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I have a theoretical lithium problem. Imagine my camper had a lithium battery already. It is sitting in the sun and gets driven about but the habitation electricity side has not been used for 6 months. Lets assume I can set my b2b ring rscdc30 to the the same absorbtion as float at the voltage recommended in the video post 226. Even then the battery has been nearly fully charged for 6 months when it would have been better at 40% SOC. I cannot disconnect my battery because I think the b2b needs a battery connected. I would have to disconnect that. Then if I knew this would happen I would run the fridge until the Soc was optimal.
I imagine an automatic system would be complicated?
As it turns out it would be better still if my solar had charged my starter battery over winter as I had to replace the starter battery due to making no long runs. Again I think The ring does not do that. Although clearly it is possible as I think my previous ctek d250s did.
 
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