Battery/BMS compatable with Genasun Boost

[Graham123]

Hello. I'm buying a new battery for my ebike, and I want to experiment with charging it via a small (100-200w) solar panel (while I'm riding). The Genasun GVB-8, or less expensive boost MPPTs, claim to be able to boost the voltage from a small panel to charge a typical ebike battery. I am aware that it will be a slow charge, and an unweildy panel, but I have applications where I think it may be worth it (A roof on an e-trike for example). I want to use a lifePO4 battery for safety reasons.

My question is - what lithium packs, or specifically, what BMS, will work for this? They can't all be designed to be used in this way. They would be using the charge and discharge port simultaneously, which I don't believe is always possible, depending on the BMS, and certainly is not what they are designed for. I've seen some sellers specifically state not to do that. However, I know someone's figured it out, because you see these builds around. How can I find out what specific battery hardware or battery brands are available which will work for this? Thanks

 

[jonyjoe303]

chargery bms8t (or bms16t/bms24t), they use contactors on both the charge and discharge. I use the chargery bms8t on my 4s lifepo4, its the only bms I would use with solar, it would be compatible with any charge controller, just put the contactor (or 12 volt 30 amp automotive relay which is what I use) between the panel and controller. This bms also can do regenerative charging when your braking.

The one I have came with a 100amp shunt but they sell with larger shunts(use to measure amps in/out), I payed 86 dollars for the chargery bms8t. The bms is fully programmable voltage cutoff points, 1.2 amp balancing, 2x temp probes, audio alarms, SOC display.

The reason this bms is good for solar is because using contactors(mechanical relay) it cuts power completely off when the battery is fully charge. A regular bms uses mosfets (electronic relays) to cutoff power, these electronic relays leak voltage causing the solar controller to produce voltage surges.

In this picture it doesnt show the shunt or connecting cables(included with unit) you have to supply your own contactors (mechanical relays) I use 4 dollar 30 amp automotive relays.


As far as lifepo4 being safer, I had one lifepo4 26650 cell catch on fire on me when I shorted it out. They are too heavy and take up too much space. I made a 9ah lifepo4 pack that takes almost as much space as a 40ah li-ion pack. In a collision where a lifepo4 gets punctured it will catch on fire just like a li-ion.

 

[amberwolf]

They would be using the charge and discharge port simultaneously, which I don't believe is always possible, depending on the BMS, and certainly is not what they are designed for.

THe only problem I can think of with doing this is if the charge and discharge FETs (if separate) use a common heatsink, and if that heatsink is incapable of shedding the heat from both charge and discharge currents at the same time. A larger heatsink or active cooling would fix that.

A BMS that uses contactors (relays) instead of FETs wouldnt' ahve this problem.

Since discharging while charging doesn't actually charge the battery anyway, but rather just passes the charge current thru to the discharge port, (assuming discharge current is the same as or higher than charge current), there's nothing about the battery itself that would matter for that.

When discharge current is lower than charge current, then some (whatever the difference is) of the current is able to charge the cells.

 

[john61ct]

And LFP is far less likely to cause any safety issue, even when punctured. I believe LTO is the other, the remaining LI really are prone to thermal runaway if you don't know what you're doing, especially once past the usual EoL point.

Anecdotal "I've been charging my packs under my kid's bed overnight for years no problem" notwithstanding

 

[billvon]

My question is - what lithium packs, or specifically, what BMS, will work for this? They can't all be designed to be used in this way. They would be using the charge and discharge port simultaneously, which I don't believe is always possible, depending on the BMS, and certainly is not what they are designed for. I've seen some sellers specifically state not to do that. However, I know someone's figured it out, because you see these builds around. How can I find out what specific battery hardware or battery brands are available which will work for this?

I have done this twice. No problems. Both times I used batteries with a single connector, but there's no reason it won't work with separate connectors (charge/discharge.) You are talking very low charge rates.

 

[Graham123]

chargery bms8t

Great BMS. Possibly a little bulky to have on a bike. However, to use this, would I have to build my own battery or work with a custom manufacturer? Or could I somehow replace the BMS of a commercial pack without disassembling the whole battery? It sounds like the charge contactor is the most important feature. Might there be a smaller, less expensive way to insert a contactor in series with the charge controller and have it be activated (closed) by a voltage threshold read from the battery? That way, any mosfet BMS could be used while still having some reassurance against leakage.

I had one lifepo4 26650 cell catch on fire

Amazing. I had never heard of that happening with LifePO4. I'm still confident that it's much less likely. The size/weight is not a huge issue in my case.

I just received confirmation from Sun-Ebike that their battery which I was looking at can work with the Genasun while riding. They gave me a specific voltage and a maximum current recommendation. Still concerned about the FETs leaking.