Sceptic (Twyfelaar) is going to build own LiFePo battery to replace AGM

[Jonatan]

I would chuck out the over current also. The only reason it is there is to protect the BMS, not the battery. That's why most Lithiums cant be used as starter batteries.

Historically, fuses have served us very well for over-current protection, but I thought to give the BMS a little bit of credit . . . . . after all, BMS somehow earned its reputation as our saviour from horrifying lithium fires. . . . .

BMS promotes itself as a cell balancer, but I am realising it only does this under certain conditions, and could actually make things worse for cells at the limits of their operating range (very full, or very empty). I want to see cell balancing in the middle 80% of the operating range, but they don't do that. (After discharging my battery by 15%, the cells are in perfect balance again.)

My one will be used as an emergency starter battery . . . . with the BMS bypassed. Take off lid and have connection points for the jumper leads inside. Only a DIY battery will give this facility.

[Jonatan]

Have you asked the supplier about swapping out the weak cell because that's not an insignificant difference and weak cells generally don't get better with time.

It is not really a weak cell, it delivers its share of the 100 Ah, but on a slightly different graph to the other three. For the middle part of the graph it is exactly the same as the rest.

[hatjohan]

I would chuck out the over current also. The only reason it is there is to protect the BMS, not the battery. That's why most Lithiums cant be used as starter batteries.

[Fluffy]

Historically, fuses have served us very well for over-current protection, but I thought to give the BMS a little bit of credit . . . . . after all, BMS somehow earned its reputation as our saviour from horrifying lithium fires. . . . .

BMS promotes itself as a cell balancer, but I am realising it only does this under certain conditions, and could actually make things worse for cells at the limits of their operating range (very full, or very empty). I want to see cell balancing in the middle 80% of the operating range, but they don't do that. (After discharging my battery by 15%, the cells are in perfect balance again.)

My one will be used as an emergency starter battery . . . . with the BMS bypassed. Take off lid and have connection points for the jumper leads inside. Only a DIY battery will give this facility.

For a BMS to be able to disconnect the battery it needs a Silicon Switch - in effect a MosFET transitor or similar, in series with the battery.. The BMS max current is determined by the size of this Switch. High current (300-600Amp) devices are hellish expensive.

So if you look at the range of BMS's available, the higher the current the higher the price.

I have a Lithium pack I made for my Motor-Bicycle. I do exactly what you do. Permanently wired so that starter is directly across battery, but charge and run power is via the BMS

[Jonatan]

Have put on longer (copper pipe) posts for when jumper cables need to be used (hopefully never). The short bit of trunking in the middle is to prevent the jumpers from touching the posts under there,




All of it still fits easily into the box. this box is as wide and as high as the classic 102 Ah AGM battery, and a bit shorter. Will easily fit into same space in vehicle



Tomorrow's job is to loosely secure the battery assembly in the case and connect the main leads to the posts in the lid of the case.

[Jonatan]

If I ever had to do this again, I would look for a more economical BMS. This DALY BMS I have now is smoke and mirrors, built in the style of gamer's computers. Two fake plastic fans is really the cherry on top!

This BMS looks quite adequate, as an example, and a heck of a lot cheaper. The glossy paint still a bit suspicious . . . .

[SAND]

Any suggestions for a 4S (12v) BMS with Bluetooth monitoring?

[Jonatan]

This project has gotten as far as I set out to do: Replace 102 Ah AGM with a lithium to fit in the same space . . . . .



The completed battery has a usable 97 Ah, or thereabouts weighs 10 kg, costs just over R5000 and needs a babysitter. (must not be kept fully charged, for example)

The old lead battery has a usable 50 Ah, weighs 25kg, costs R2100 and is fit and forget.

The lithium (if nursed) supposedly will give much longer life than the lead, but I need to get my own real world experience of this.

Still figuring out last problem: My auxillary battery is in my daily driven vehicle with a semi-permanent fridge. We often go away a couple of days on the spur of the moment. Having a permanently charged lead acid battery worked perfectly. But with lithium, if I want longest life, I must keep the battery at less than full . . . . trying to figure out how to do this in a daily driver vehicle . . . .

[Jonatan]

Calibrated my voltmeter with a controlled discharge over 5.5 hours. Disconnected battery every hour and let battery rest / recover for 10 minutes before measuring voltage:



Essential to measure voltage at rest only (disconnected). I was seeing about 0.2V drop at 17 Amp draw.

Typical voltmeters only have one decimal place indicated (as above). But that is okay as a SOC indicator with about a 20% range of accuracy.

Of course the built-in bluetooth systems give more decimal places, but I would only trust them if I have done the same test as above.

[Jonatan]

Disconnected the charging system and let the battery run just the 40 liter NL fridge. Was already cold. Ambient temp mid 20's C. The fridge has run for about 2.7 days. (The 3-yr old AGM battery only ran the fridge for about 12 hours and was due for replacement)

[Jonatan]

It is now 6 months down the line . . . . this "homemade" battery has spent its time in my car (daily driver) as the auxiliary battery, charged by a WRND DCDC, permanently powering a couple of USB outlets and for a couple of months powering a 50 liter fridge. Yesterday I removed the battery for a check and drained it with 4 headlight bulbs until the BMS disconnected it. Last night I charged it with a smart charger and saw this morning that 96.8 AmpHours had been pumped back into the battery.

I am a very happy man - no longer a twyfelaar! Despite one cell being badly out of balance on receipt, the cells have stayed balanced well enough. Taking it out of the car made me appreciate the huge difference in weight. During the last loadshedding I helped a lot of friends and family with their 100 Ah lead acid batteries, which are much heavier and in reality only give 50 Ah. I now think the lead acids are the finicky ones - I can't just drain them until they die. Downside is the cost difference - a huge factor. But one can save a lot by building the battery yourself if you have a moderate understanding of electrics.

Snag is, with a quick online search this morning, nobody in SA has economical lithium cells in stock. (I wanted to look at building about 300 Ah 12V for my home UPS.)

[propofol]

[QUOTE=Jonatan;4920045]

Where did you buy the empty battery case for lack of a better word?

[Jonatan]

[QUOTE=propofol;4921077]

Where did you buy the empty battery case for lack of a better word?

I thought that I put all my notes in this thread- did you read through it all?

[propofol]

[QUOTE=Jonatan;4921106]

I thought that I put all my notes in this thread- did you read through it all?

Thx! Found it. Must have missed it the first time round.