Heading Over To The Dark Side

Please feel free to describe your motorhome, campervan or caravan. (make, model, length etc)
Mrcoolabah1au
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Re: Heading Over To The Dark Side

Post by Mrcoolabah1au »

Yes being following this thread and others ⚡️On fb and other sites 🤔
Coolabah1au
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jon_d
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Re: Heading Over To The Dark Side

Post by jon_d »

Hi NP, if you use longer cables to the aux pack than those from the main pack that powers everything then they will just remain topped up. If the cables are shorter however, the 120 Ah pack will become the supply pack and the 480Ah battery will just be an aux tag on that offers very little to supply load until the 120Ah pack is completely drained. You can try to get the resistance between the load and the battery packs identical and that will help, but the battery pack with the least internal resistance will still be the main supply until virtually exhausted causing the resistance to increase, then the other pack will take over. Eventually the pack with the lowest resistance will have been tortured to the degree that the internal resistance increases till it is the same as the other pack, then they will sort of work together.
I was thinking about this......

If you have a battery pack cabled to a another pack and then to the load. Then the above is correct.

But, if you have the 2 packs cabled with equal length of cables to the load, then the effective internal resistance (battery and cable) is the same. In theory, the two packs should share the load. Obviously,minor difference in cells and cable resistance would still influence the balance between the two packs.

Which is how I have cabled my AGM's to the inverter.
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T1 Terry
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Re: Heading Over To The Dark Side

Post by T1 Terry »

Resistance is resistance, doesn't matter if it is cable resistance, terminal crimping or connection resistance or internal battery resistance, it all adds together to make the total flow path resistance figure. Electrons are lazy and will flow through the least path of resistance every time. Lead acid batteries internal resistance changes quite dramatically as its state of charge reduces, so a sort of balance will occur over time. Lithium batteries don't follow the same rules, anywhere between 99% state of charge and roughly 40% state of charge the internal resistance is virtually the same, so one battery will supply the majority of the current while the others do very little, then the next lowest resistance path will start to take some of the load and so on. Recharging is the reverse. The battery with the lowest total resistance path gets hammered, then the next and so on. Once the hammered battery is damaged to the point its resistance increases, not only will the high load move to the battery with the next lowest path of resistance, that battery will also be later reaching the fully charged state or even have lost capacity so it reaches the cut off voltage well before the other batteries in the parallel connection string. This battery will also become a load across the other batteries when no charging current is being seen on the system and it will gradually drag the other batteries down to its own reduced capacity ... the beginning of the death cycle that occurs with any chemistry battery pack built up using parallel connected batteries. This even happens within a battery cell, it only requires one short between a positive and negative plate to flatten that whole cell. If you can't isolate that cell then the whole battery voltage drops and becomes a parasitic load that will destroy the whole battery pack if not caught soon enough.
How would you determine if a battery had a damaged cell when you can't access the individual cell voltages or even the individual battery voltages?
I'll see if we can video the demo at Stone the Crows so we can post it up on You Tube to show just what happens. I might end up with death threats from the drop lithium battery crowd because it would seriously damage their future sales to anyone who viewed the video, but after the "go for the throat" attacks I received from forums over the last 8 yrs I'm fairly confident I'll get through another threat attack :lol:

T1 Terry
A person may fail many times, they only become a failure when they blame someone else John Burrows
If we have data, let’s look at data. If all we have are opinions, let’s go with mine. – Jim Barksdale, former Netscape CEO
nut17
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Re: Heading Over To The Dark Side

Post by nut17 »

I must run across the cells with a meter to check that this installation in my mates "Explorer" motorhome on a 2017 Ford Ranger to make sure the pair of paralleled 12 volt 100 AH LiFePO4's are still remaining equal. This battery installation was completed by AA Solar in Silverdale, near Auckland in 2017. The shape and size of the space for the batteries would not accommodate a single 200 AH unit.
Cheers
Chris

IMG_4153.JPG
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2020 Winnebago ILUKA on IVECO Daily 50 C. 1200 w Solar - 400 AH LiFePO4
3000 w inverter. 195 lt compressor fridge NZMCA # 15589
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T1 Terry
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Re: Heading Over To The Dark Side

Post by T1 Terry »

nut17 wrote: Sun May 26, 2019 6:19 pm I must run across the cells with a meter to check that this installation in my mates "Explorer" motorhome on a 2017 Ford Ranger to make sure the pair of paralleled 12 volt 100 AH LiFePO4's are still remaining equal. This battery installation was completed by AA Solar in Silverdale, near Auckland in 2017. The shape and size of the space for the batteries would not accommodate a single 200 AH unit.
Cheers
Chris


IMG_4153.JPG
A meter won't tell you anything except cell voltage and we know lithium batteries hold a fairly constant voltage anywhere between 95% SOC and 20% SOC. They will even hold a very close voltage all the way down to 0% SOC if the load is small. The difference between 0%SOC and completely drained is the tricky bit because the voltage drops suddenly when there is absolutely nothing left in the cell.
The only way to tell if they are load sharing is to capacity test each 12v battery under a 0.5CA load test, 50 amps for 2 hrs on a fully charged battery.

We did a demo at the Stone The Crows get together and put the output from each of 4 100Ah batteries connected in parallel ith a Victron 700BMV between each. With a 200 amp load we wanted to show that each battery would not deliver an even share at 50 amps from each battery. The first battery supplied the most, the second battery supplied around 20% and the other two supplied nothing once the first 2% was drawn off. After only a few mins there was a 20% difference between the highest and lowest battery SOC, yet all 4 batteries were at the same voltage.
Basically, the first battery get worked to death until a cell fails, then it takes out the other 3 batteries because they are constantly trying to bring the first battery up to the same voltage. With one cell not holding voltage the other 3 cells get an over voltage charge until they are seriously damaged, then the whole battery drains the other three. With no cell voltage monitoring you would see any signs of trouble until it was too late.
Exactly the same thing happens with lead acid batteries in parallel, one cell fails and drags the other batteries to their death if you aren't quick enough to catch it.

As far as the space not being sufficient for a 200Ah unit, why? 2 x 100Ah @ 3.2c cells in parallel makes a 200Ah cell @ 3.2v, so each 4 cell battery would become a 200Ah 6v battery, 2 linked together makes a 200Ah 12v battery. Same number of cells without the problem of one battery doing all the work. You would think AA Solar would understand the basics eh????
Interesting to see they use Sinopoly cells, I believe Enerdrive use them as well. We had capacity loss issues with one set we installed for Tom and Carmel and we swapped them out for a bigger battery pack built out of Winston cells and gave them a pro rata refund on the Sinopoly cells.

T1 Terry
A person may fail many times, they only become a failure when they blame someone else John Burrows
If we have data, let’s look at data. If all we have are opinions, let’s go with mine. – Jim Barksdale, former Netscape CEO
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