T3TRIS
New Member
- Joined
- Feb 22, 2022
- Messages
- 28
Hey everyone,
I've learned quite a bit from browsing other people's posts and builds (especially from Dan-O's post) so I figured I'd share my own attempt at building a LiFePO4 battery system.
The goal is to add an auxiliary battery to our beloved 4WD 1987 Volkswagen Vanagon, which has no auxiliary battery system built-in as it was not originally a Westfalia camper. Without getting into too many details, I'm trying to design a system with an oversized capacity so that we don't have to rely on solar panels (though I'll probably install something at some point), the irony for a DIY Solar Power Forum... In any case, with 320Ah, the idea is that we'd have enough power to enjoy up to a week of camping which would drain the battery about 50%, and with a beefed up charging system (upgraded alternator + 50A DC/DC charger), the battery would recharge within the first few hours of driving. This makes it so that we don't have to think about solar panel placement when we camp.
Anyhow, so far I've built the battery compression box with the Overkill BMS using cutting boards and 1/8" aluminum pieces and other hardware. I used 5/16 threaded rods and these steel shelving rack brackets from Ace Hardware in which the 5/16 locking nut nests itself (self tightening basically). They also had these springs that are rated for 200lbs. Apparently, we're all aiming for 12PSI of pressure, which means 168lbs on each rod for a 6"x7" cell size. The cells sit on a tray that is not connected to the side pieces so that they can move independently when the cells swell and contract a little. The cells are separated by a heavy duty piece of silicone. The threaded rods are sheathed with tubing. The bus bars are flexible bus bars from eBay.
This was the first iteration, since then I've made a few changes. I've flipped the BMS around to prevent the positive and negative wires from overlapping.
I also tested these compression springs and the first one collapsed with less that 70lbs of weight so I upgrade the hardware to 3/8 and die springs from McMaster Carr (9573K63 Blue Medium Load, 1" diameter, 1.25" long, 225lbs max load, 450lb/in spring load). I compressed the springs to around 1" or about 110/120lbs with the cells depleted.
In the photo, you can also see a couple 12W heating pads though I haven't quite figured out the details for this yet. I have a small 12V programmable thermostat for this (35mA while at rest) and plan on wiring it to trigger only when the alternator is running.
I also added a 150A block fuse directly on the cell's positive terminal.
I still have to figure out the heating part, how to secure the battery to the van and making a top for it to prevent shorts.
Here's the wiring that this battery will power.
Lastly, I'm trying to figure out how to setup the BMS parameters to stay on the conservative side of charging and discharging since this system is oversized. By default, the Overkill BMS stops charging when the battery reaches 14.6V or any one cell reaches 3.65V, and stops the discharge when the battery drops to 10V or any one cell to 2.5V. I've definitely noticed that cell #3 drops down much quicker than the others and cell #1 reaches 3.65V first (though less dramatically than cell #3 on discharge). In the middle, they are all within 0.03V though. The cells were top balanced in parallel to 3.65V until the 10A power supply was pushing less than 0.2A.
I wondered if I can set the BMS to stop charging when a cell reaches 3.5V or drops to 2.8V (and 14V/11.2V) or if that was bad idea or unnecessary. I noticed the delta between cell #3 on discharge augments quickly. By the time it reaches 2.5V, the other cells are still around 0.35V higher and within 0.15V of one another.
I've read other posts about this but haven't quite found if a "conservative" approach is beneficial, though I've seen people mention "low stress charging" or something like that.
Any input on the BMS settings would be appreciated.
I've learned quite a bit from browsing other people's posts and builds (especially from Dan-O's post) so I figured I'd share my own attempt at building a LiFePO4 battery system.
The goal is to add an auxiliary battery to our beloved 4WD 1987 Volkswagen Vanagon, which has no auxiliary battery system built-in as it was not originally a Westfalia camper. Without getting into too many details, I'm trying to design a system with an oversized capacity so that we don't have to rely on solar panels (though I'll probably install something at some point), the irony for a DIY Solar Power Forum... In any case, with 320Ah, the idea is that we'd have enough power to enjoy up to a week of camping which would drain the battery about 50%, and with a beefed up charging system (upgraded alternator + 50A DC/DC charger), the battery would recharge within the first few hours of driving. This makes it so that we don't have to think about solar panel placement when we camp.
Anyhow, so far I've built the battery compression box with the Overkill BMS using cutting boards and 1/8" aluminum pieces and other hardware. I used 5/16 threaded rods and these steel shelving rack brackets from Ace Hardware in which the 5/16 locking nut nests itself (self tightening basically). They also had these springs that are rated for 200lbs. Apparently, we're all aiming for 12PSI of pressure, which means 168lbs on each rod for a 6"x7" cell size. The cells sit on a tray that is not connected to the side pieces so that they can move independently when the cells swell and contract a little. The cells are separated by a heavy duty piece of silicone. The threaded rods are sheathed with tubing. The bus bars are flexible bus bars from eBay.
This was the first iteration, since then I've made a few changes. I've flipped the BMS around to prevent the positive and negative wires from overlapping.
I also tested these compression springs and the first one collapsed with less that 70lbs of weight so I upgrade the hardware to 3/8 and die springs from McMaster Carr (9573K63 Blue Medium Load, 1" diameter, 1.25" long, 225lbs max load, 450lb/in spring load). I compressed the springs to around 1" or about 110/120lbs with the cells depleted.
In the photo, you can also see a couple 12W heating pads though I haven't quite figured out the details for this yet. I have a small 12V programmable thermostat for this (35mA while at rest) and plan on wiring it to trigger only when the alternator is running.
I also added a 150A block fuse directly on the cell's positive terminal.
I still have to figure out the heating part, how to secure the battery to the van and making a top for it to prevent shorts.
Here's the wiring that this battery will power.
Lastly, I'm trying to figure out how to setup the BMS parameters to stay on the conservative side of charging and discharging since this system is oversized. By default, the Overkill BMS stops charging when the battery reaches 14.6V or any one cell reaches 3.65V, and stops the discharge when the battery drops to 10V or any one cell to 2.5V. I've definitely noticed that cell #3 drops down much quicker than the others and cell #1 reaches 3.65V first (though less dramatically than cell #3 on discharge). In the middle, they are all within 0.03V though. The cells were top balanced in parallel to 3.65V until the 10A power supply was pushing less than 0.2A.
I wondered if I can set the BMS to stop charging when a cell reaches 3.5V or drops to 2.8V (and 14V/11.2V) or if that was bad idea or unnecessary. I noticed the delta between cell #3 on discharge augments quickly. By the time it reaches 2.5V, the other cells are still around 0.35V higher and within 0.15V of one another.
I've read other posts about this but haven't quite found if a "conservative" approach is beneficial, though I've seen people mention "low stress charging" or something like that.
Any input on the BMS settings would be appreciated.