DIY Lithium

Harryw

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I have a similar fall back arrangement. The small space under my car is more square in plan than 4 in line. I have 8 cells ordered. They may fit in 2 batteries of 4 like Harry and Phils layout but If not I plan each battery of 4 to be 2 pairs of cells joined by a busbar, with a heater pad between, the cell of one battery made of 2 pairs in an L shape, hopefully the join being by a standard bus bar. The 2 L shapes then fit together into a square around a core of timber to take up the small empty space in the middle. The whole 8 cells then shrink wrapped and put in an enclosure with their Bms's. As long as the 2 L shaped batteries have equal length positive and negative tails to each other I would think that will work as there is symmetry between the 2 batteries.
I would not have thought that the positives need to be the same length as the complicated negatives through the BMS. As long as the 2 batteries are the same overall resistance, it should work.
How big is your space? As 2 4cell packs would drop into a 400x300mm Euro type crate for under a tenner?
 

1 Cup

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I'm looking at cells, and it's 70 x 170 x 250 mm the 250 is so you can fit studs and remove if necessary. Bus bars at 330mm that will do ! three bats at £50 each or a meter.
Im going four up and not tetruss now. Still only 1.25 m from van floor 125kg includi g victron stuff. The water tank should balance it out.
 

Harryw

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Just looked at Euro boxes, it refers to external dimensions so a 60ltr 600x400mm will be needed to fit 8 cells with associated BMS(s), 320mm high too for ease of access and cable management.
It’s a cheap and clean way of packaging them, removable as a single unit too, but I suppose available space will dictate in the end though.
 

Derekoak

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How big is your space? As 2 4cell packs would drop into a 400x300mm Euro type crate for under a tenner?
That is too big, my cells are 8 x 100amp hour cells from memory (I am away in Scotland) they are 35mm x 135mm x 210 high (may or may not include threaded bolts/ rods. The space under the car is about 270 deep 240 wide 270 long. The square 2 x Ls array fits in 210 x 210 x 210 with space somewhere for 2 Bms.
 

wildebus

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I'm still wondering if I should make two 12 volt batteries, each with a BMS or one 24 volt battery? Would the two BMS's talk to each other? If not it would be a pain.
if they are independant batteries, I think it would be dependant on the BMS but it would be rare. you do get a few batteries with interconnecting comms cables but generally high-end gear.
Making a 24V one probably the way to go?
 

Harryw

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That is too big, my cells are 8 x 100amp hour cells from memory (I am away in Scotland) they are 35mm x 135mm x 210 high (may or may not include threaded bolts/ rods. The space under the car is about 270 deep 240 wide 270 long. The square 2 x Ls array fits in 210 x 210 x 210 with space somewhere for 2 Bms.
Sorry, struggling to envisage a 210mm square package made up of 8 cells at 35x135mm ea. Do you have a sketch?
 

wildebus

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Sorry, struggling to envisage a 210mm square package made up of 8 cells at 35x135mm ea. Do you have a sketch?
If you put 6 cells 35mm deep against each other it makes a package 210mm deep x 135 long.
The last 2 cells are sideways against the 210 length and you end up with a final package 210 x 205 (and 210 high) with a void in the corner of 75mm x 70mm x 210 high - in which I presume you drop in the BMS?
Those dimensions assume no/tiny spacers between each cell of course and very little tolerance room.
 

Philip Tomlinson

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Today I took a screenshot from the Victron VRM of my lithium battery voltage with my engine running. My van is a 2013 Ducato with the standard 150A bosch alternator. My solar is switched off. The voltage you can see is the charge voltage from starter battery via a victron Cyrix-li relay. Please note there is only 9w going into the batteries which will be the self consumption of the four BMS units with their bluetooth radios on.

29C8B752-D779-4D66-B5AF-5199EFD5FE3C.png
EB30A268-4992-46F8-B30F-3DF658449A9E.png
 

Harryw

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I presume the Lifepo4 are already full if not drawing? I don’t have the VRM so can’t use Cyrix Li but did consider a Cyrix CT but as it’s bidirectional the Lifepo4 would always be charging the van battery due to trigger levels above 12.8v. So will probably do z as B2B soon….

One thing I note about your MPPT settings, whilst you’ve gone with user defined battery settings I’d consider setting the temp compensation off, ditto equalisation, not really applicable for li charging as I understand it. Same briefly set it to expert mode and setting the pk to unity…
 

Philip Tomlinson

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I presume the Lifepo4 are already full if not drawing? I don’t have the VRM so can’t use Cyrix Li but did consider a Cyrix CT but as it’s bidirectional the Lifepo4 would always be charging the van battery due to trigger levels above 12.8v. So will probably do z as B2B soon….

One thing I note about your MPPT settings, whilst you’ve gone with user defined battery settings I’d consider setting the temp compensation off, ditto equalisation, not really applicable for li charging as I understand it. Same briefly set it to expert mode and setting the pk to unity…
It is a cyrix li ct, you don’t need any extra gear to use it.
 

Harryw

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It is a cyrix li ct, you don’t need any extra gear to use it.
Yes, I’m a bit confused as the description says it analogous to the normal ct? In which case I thought it was still bidirectional and disconnect is not activated until below 13.2v which the Lifepo4 stays above hence it supports the van battery ….?
 

Philip Tomlinson

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Yes, I’m a bit confused as the description says it analogous to the normal ct? In which case I thought it was still bidirectional and disconnect is not activated until below 13.2v which the Lifepo4 stays above hence it supports the van battery ….?
The Li is the LifePo4 version, it is bidirectional and the voltages are correct for lead-acid starter to LifePo4 leisure batteries. It will disconnect if the voltage falls below 13.3v for more than 10 seconds or immediately if the voltage falls below 13.2v. So when your battery voltage drops to 13.2v it will stay connected for 10 seconds.

6076DB33-859C-47F8-9029-078158670B99.jpeg
 

xsilvergs

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I don't have DIY Lithium but Ecotree, long before I got the Lithium I had decided on a B2B.

I have Absorption set to 14.3v and Float to 13.5v for the B2B and both MPPT's.

Watching the alternator voltage variation whilst driving and the Lithiums voltage as the 30A B2B limits its output to ~400 watts in Bulk mode surprised initially.

Even though at present I have only a single B2B the solar still contributes and see up to 60A charge rates. Tree shadows don't help though whilst driving.

I did consider the Cyrix but I just couldn't convince myself, for a few reasons and I don't get any discharging of the Lithium back to the starter battery.
 

Philip Tomlinson

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I don't have DIY Lithium but Ecotree, long before I got the Lithium I had decided on a B2B.

I have Absorption set to 14.3v and Float to 13.5v for the B2B and both MPPT's.

Watching the alternator voltage variation whilst driving and the Lithiums voltage as the 30A B2B limits its output to ~400 watts in Bulk mode surprised initially.

Even though at present I have only a single B2B the solar still contributes and see up to 60A charge rates. Tree shadows don't help though whilst driving.

I did consider the Cyrix but I just couldn't convince myself, for a few reasons and I don't get any discharging of the Lithium back to the starter battery.
You can of course use the enable pin on the cyrix and connect it to an ignition live so that it is disabled when the ignition is turned off. I would not suggest this because then you stop the cyrix maintaining your starter battery from your solar.
 

xsilvergs

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You can of course use the enable pin on the cyrix and connect it to an ignition live so that it is disabled when the ignition is turned off. I would not suggest this because then you stop the cyrix maintaining your starter battery from your solar.
One function I don't have (at the moment) is the ability to maintain the starter battery from the solar. I was using an Ablemail AMT12-2 battery maintainer but its default Lithium setting didn't work for me. Hopefully they will reprogram it or I'll just build my own.
 

Harryw

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Slightly wine and beer impaired last night, but yes totally get that.
The point I was badly making was that a Lifepo4 cell pack will be at 85% at 13.3v and 70% 13.2v. Ergo if the pack is fully charged when you shut down the engine the Lifepo4 will continue to back feed the starter battery until 2 minutes after dropping to 13.3v (85%).
You could consider it a bonus though.

out of interest do you know what the limit of draw by the Lifepo4 would be in this configuration, as it’s CV @14.4v max?
 

Philip Tomlinson

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Slightly wine and beer impaired last night, but yes totally get that.
The point I was badly making was that a Lifepo4 cell pack will be at 85% at 13.3v and 70% 13.2v. Ergo if the pack is fully charged when you shut down the engine the Lifepo4 will continue to back feed the starter battery until 2 minutes after dropping to 13.3v (85%).
You could consider it a bonus though.

out of interest do you know what the limit of draw by the Lifepo4 would be in this configuration, as it’s CV @14.4v max?
I am not sure where you got those voltages from. 13.2v is the resting voltage of lifepo4 battery after about an hour (3.3 x4).

limit of draw of current?
 

Derekoak

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Sorry, struggling to envisage a 210mm square package made up of 8 cells at 35x135mm ea. Do you have a sketch?
20210516_084012.jpg
Thanks for thinking about it.
Edit Wildbus's layout could be made to work at about 227 x 210 if it fitted a enclosure better. I prefer the symmetry of the 2 L shaped batteries of my layout but I need to find space for the BMS 's somewhere. There are 4mm heat pads between each pair of cells and 1mm spacers elsewhere.
A corner for the Bms of Wildbus layout, might be worth the extra. It would need a rigid box with bms within, in the corner to keep the shrink wrap working.
 
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Derekoak

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I don't have DIY Lithium but Ecotree, long before I got the Lithium I had decided on a B2B.

I have Absorption set to 14.3v and Float to 13.5v for the B2B and both MPPT's.

Watching the alternator voltage variation whilst driving and the Lithiums voltage as the 30A B2B limits its output to ~400 watts in Bulk mode surprised initially.

Even though at present I have only a single B2B the solar still contributes and see up to 60A charge rates. Tree shadows don't help though whilst driving.

I did consider the Cyrix but I just couldn't convince myself, for a few reasons and I don't get any discharging of the Lithium back to the starter battery.
Which 30 amp b2b do you have? Which mppts?
 

Derekoak

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The Li is the LifePo4 version, it is bidirectional and the voltages are correct for lead-acid starter to LifePo4 leisure batteries. It will disconnect if the voltage falls below 13.3v for more than 10 seconds or immediately if the voltage falls below 13.2v. So when your battery voltage drops to 13.2v it will stay connected for 10 seconds.

View attachment 97598
So the cyrix li ct is a smart split charge relay which will also allow charge to flow from solar to the starter battery? This is all you use to get significant charge from your alternator to your Diy Lithium leisure battery?
Can you set the max charge current to protect the alternator?
 
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