I’m building two boards based on Pacificmeister’s design. Batteries are now settled thanks to forum posts last week. Thanks to all who helped out.
This week I’m trying to figure out BMS. Seems some people do and some people don’t. I have a balance charger or two and I have low voltage alarms but I can’t decide whether I should go for BMS. I like the functionality but a couple of things. First of all, cost. As I said, I’m in this for two boards so there’s that cost. Add to it that I expect the family will want many battery packs so we can stay out for a while. So, each battery pack will add up.
Second, I don’t know what to look for. I’m not an electronics guy. I’m building 10s6P battery packs to run in parallel for each board.
Honestly, I don’t mind the extra cost. I’m already neck deep in this. So what’s some extra bucks. But, I def can’t afford the fancy BMSs that have Bluetooth and all that jazz.
Any advice appreciated. ESP, recommendations for a good bms for that battery pack.
Finally, I really appreciate all the info and community of this group. Technology is truly wonderful these days. It is enabling such cool things.
Personally I prefer the “no BMS, with a low voltage alarm” method.
Pro’s to no BMS:
One less thing to fail (this is huge reason for me. I have had many BMS issues on electric skateboards over the years. If the BMS thinks something is wrong it shuts off the battery. I don’t wanna swim in more often then needed.)
More reliable board (as mentioned above)
Give user more control (not having a BMS means you can decide how low you wanna run your battery, let’s you balance charge more accurately, and stop charge at lower user specified voltage so batteries last longer).
Charging battery is more accurate with hobby charger.
Con’s to not having BMS:
User can damage battery (if friends and family use board, and continue to ride below safe voltage battery will be permanently damaged.
More liability (potential fire if user error during charge). You must only charge the battery via a method that performs balance charging! This is very important.
Must have hobby charger with balance charging capability.
No BMS usually requires two separate batteries that you must manual connect in series for use, and charge separately. So, more plugs required on waterproof box.
I have removed the faulty BMS on several electric skateboards and converted them to no BMS with low voltage alarm. It is a reliable, safe system as long and user is careful not to over discharge, or over charge the batteries. And this means you must always balance charge the battery!!!
Hi! I’ve built electric cars, ATV’s boats and efoils. Some use BMS some dont. My take is: If you get HIGH quality cells AND make sure that they are charged in parallel before building your pack, you can rely on a low voltage cutoff from the ESC/controller.
With the efoil I like to test the voltage cutoff by setting it at a higher voltage and when the esc cuts off I’ll use my charge to give me a SOC reading. I’ll tune the cutoff until it gives me approx 10-15% SOC.
I would never recommend using a charger in fast mode, always do balance charge to make sure the cells are leveled.
With voltage cutoff set conservatively where 10%-15% of battery capacity remaining should result in a end voltage just above 3.0v per cell. With quality cells that are properly balance charged my end voltage variation between cells is always within 0.1v.
My low voltage alarm is just added protection, as it monitors each cells voltage. But, as @Riwi says, you could get away without it.
Another great benefit of the low voltage alarm is you can set it a bit higher and it works as a “reserve fuel” indicator. On my longer rides this is nice to have. When the alarm goes off I know I have 10 minutes, or about 1.5 mile range remaining before the ESC starts to restrict power.
Still asking for the datasheet and specs. But I don’t think their agent knows what she is talking about. i asked “What is the purpose of the contactor coz BMS has it’s own safety features right? ” She answered “It’s for the bluetooth”.
The Pro’s
The contactor will have a very low internal resistance and therefore much less internal losses at high currents compared to the standard MOSFETs.
The connector load will truly be disconnected rather than “disconnected” by MOSFETs.
Cheaper than MOSFETs
The Con’s
The mechanical contactor, contact points eventually (depending on its abuse) will wear out or make pore contact after some time with unreliable connections as a result. This can lead to failing ESC’s if the connection is just bad enough especially with regenerative braking.
No precharge, if used with a capacitive load every time the relay will close a spark will form inside the relay. This eventually will result in the problem stated above or a welded contactor. In a MOSFET style BMS this won’t be possible.
Disconnection at high loads or short circuits, will definitely make some nice arcs inside the relay, and have it fail within no time.
If combined with a proper automated precharge system a lot of these problems will be solved. But I don’t think that’s inside this BMS.
Hi, I’m going to build a 12s12p battery pack with 18650 samsung 30q. If I have a bms could I charge it directly with a power supply? What power supply should I get for a fast charge? Would a 48v 10a golf cart charger work or is it overkill?
I don’t know what type of charger is golf cart charger.
For 12s lipo you need 50.4V charger.
10A surely isn’t overkill for 30q in 12p setup.
They should be able to take 36 A charge max in 12p
So for a 12S battery you hook this switching power supply up to the 60V output? And the 60V vs 50.8V on the battery isn’t an issue? I’m assuming you count on a BMS to cut the charging at the appropriate time then? If so, what BMS do you use?
i asked “What is the purpose of the contactor coz BMS has it’s own safety features right? ” She answered “It’s for the bluetooth”.