There is a lot of information on the net and I cannot and do not want to be exhaustive. However, this page is intended as a basic explanation of the project – the battery part. Part 1: What you need to know. Part 2: The building of the batteries. And always remember: “With great power comes great responsibility!”
Part 1: What you need to know about LiFePO4
- This link contains much in depth and practical information: https://marinehowto.com/lifepo4-batteries-on-boats/
- There are three types in general:
- Round cells (like 18650) that have a wrapped internal structure.
- Prismatic cells that have a block shape, as a wrap or layered like a curtain, hence prismatic ( /\/\/\ ).
- Pouches are layered without any support structure – you have to create that yourself.
Bloating of batteries
- Batteries may bulge a little and even some new batteries are not completely flat, even though they perform well.
- In principle, swelling is not intended.
- However, expansion and contraction are part of charging and discharging LiFePO4 cells.
- There is also gas formation, which is why there is a valve, usually between the electrodes.
- Just search for “LiFePO4 swelling” to get an impression. This is a document explaining it a bit more.
- My personal opinion: Lock up pouches, don’t lock up prismatic cells. Always choose a construction that can absorb expansion and shrinkage to a limited extent.
- It does not seem a good idea to me to clamp cells between two thick wooden plates with threaded rods.
- An alternative could be to make a sleeve of thin material in which they fit. But: opinions differ on this, despite my mechanical engineering background.
Prismatic cells are tested before they leave the factory. Grade A is top. If they are stored for a few months they become grade B – not a bad battery. Quality decreases alphabetically to sometimes Grade F – you don’t want these.
Before buying, you want to make sure that you do not buy garbage – probably the hardest part.
SOC – State of Charge
- The most important thing is to treat the batteries properly. This way you will have the longest enjoyment of the battery.
- Do not overcharge and undercharge, keep the voltages within limits and…
- Limit the charge and discharge current.
- Don’t store and use in hot spaces.
- Don’t charge at temperatures below 0 Celsius.
- Bad contacts are not only showstoppers, they can also cause a fire while giving the impression that everything is in order, i.e. working.
- Because this is not mentioned in so many articles and videos, I mention it here with exclamation marks!!
- Always prepare contacts.
- Dry the contact surfaces.
- Sand the surfaces with 300-grit (or a bit more) paper.
- Then clean with isopropanol.
- In wet or condensing spaces: Apply a good moisture barrier such as petroleum jelly.
- As with contact surfaces, tightening torques are rarely discussed and, look at the videos, people even do hand-tightening. Unbelievable and please, take it very seriously!
- At least use proper online tables for this.
- Do not over-tighten or under-tighten connecting screws. If you regard a threaded rod as a spring, you can make the spring a little longer by adding one or two washers before tightening the screws. For M6, a battery tightening torque of 7 Nm is mentioned, but check the manuals.
- Crimping terminals? Always double check the quality if it is not your daily activity. Use the right tools and match wire sizes with terminals. Personally I post solder crimped connections often.
- Read this: https://vanderworp.org/preserving-electrical-contacts/.