Simple range extender for Bestechpower BMS (and other also)

Skrzypas

10 W
Joined
Jun 16, 2014
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65
Location
Warsaw, Poland
Hi,

I have an 16S Li-ion system with Bestechpower HCX-D131 BMS system in 16S version. The device has standard BMS functionalties, especially protects the battery from overdischarging during use. This means if you run out of juice, it cuts off the system and you can't run the motor until you connect the charger when you reach home.

I would love to have a "one/last chance button" that would activate the BMS once to let me roll slowly home instead of pedalling on my high speed chainrings set...

I think I need one or a few parallel 18650 cells connected to a standard e-bike charger plug through a step-up converter that builds up "some voltage", probably something like 16*3,7= 59,2 V. Then when out of juice, I'd connect this setup to the charging point for 1 second and get some free low speed juice to get slowly home.

What do you think? Anybody tried? Which cheap (a few bucks on aliexpress) step-up would be ok for doing this?
 
Yeah, will not work as described.

But I’ve taken a cue from some lawn tool battery packs and merely use BMS protection for charge only. You’re riding out in the open air so if something goes fireball you can immediately deal with it much more readily than charging in your garage unattended.

You can also use a much smaller current rated BMS for charge only. The charger connects to the BMS-battery but discharge/controller power is taken directly from the battery tabs.

If your pack is relatively balanced and you have a reasonable controller LVC this arrangement can work remarkably well for a “limp home” mode. And as mentioned you don't need a more expensive high power BMS.
 
Ykick is half right. You cn use small bms based on highest charge current with contactor controlled by bms. This way you have lvc and hvc protection. then you can have manual contactor activator switch. this way when bms cuts out contactor, you could re engage it manually. Li-ion cells tend to bounce back to safe voltage when load is removed and you could limp back home for 1-2km. You have to measure cell voltages for the first time using them that way thou, to see at what voltage bms cuts out and measure again after 2km's to see if cells recovered to safe voltage just to be safe.
 
I think you'd have better luck designing a stand-alone watt-meter that flashes a light when it thinks you are approximately halfway down on the batteries usable capacity. It would vary, hard to be exact, but...maybe doable? I wish you luck with whatever you try.
 
What do you mean why? First you wont find step up converter to handle the current. Second your few cells would sag badly. The idea is totally bad. Design your pack to fit your needs. If not, carry a charger. Or you can note what mileage you get and try to plan your jurneys accordingly.
 
Skrzypas said:
I have an 16S Li-ion system
You'll get better performance and less risk of damaging cells, and probably cost less, probably even less weight and size, if you just add another set of 16 cells in parallel with the existing ones.

You could try what you want to do, by making a "12v" 3s or 4s pack, probably with at least 2p or 3p to get the current from it, then using an automotive "cigarette lighter"
inverter to try to run your charger, but you should use a true-sine-wave type, because the "modified sine wave" or square-wave output types that are all the cheap ones could either just blow up or damage your charger. You'd also need to make sure it is rated for more watts than your charger is, so the charger doesn't suck too much from it and burn it out.

If you can find a DC-DC that allows current limiting the same way a charger does it (not by just shutting off, which is how most DC-DC do it), then you could use a DC-DC directly from the little pack to the big one.

But all of these options will be larger and heavier and almost certainly more expensive than just adding another string of 16 cells in parallel with the existing pack.
 
Fit a wattmeter, which will tell you how much of your battery you've used.

Your controller normally has a LVC set above that of the BMS. That's where you need to look to control battery capacity. It would be simple enough to add a switched in resistor to one side of the voltage divider to have a two-level low voltage cut-off, which would work like a reserve tank.

All that sounds a bit unnecessary, though. A ll I do is look at the LCD, which shows when my battery is getting low, so I turn down the power.
 
Feeling completely not understood here... But ok, guys, you can monitor, plan and etc. I just wanted to have something that prevents my stupidity of using too much amps when the battery is almost empty, causing the BMS to cut off, when the battery sags too much.

Adding another 16s string to my pack? To increase the range from 100 to 110 km? Thank you, don't need.

I'll measure the initial output voltage of my charger, and try to develop the safety device of my dreams for resuming BMS far from home. During this season I would use this device 3 times. But those 3 rides I remember as peddling back home in rain or night...

Electric transportation is nice and cheap but it has an important problem: you always charge, think about charge, think what is your range, estimate, hesitate of you get there on one charge or not... If you choose just to have fun from the bike /car you'll be me with no juice in the middle of nowhere.
 
You have a 100km range with your battery and you want to pump a couple dozen watt-hours back in once it's dead? Thats like carrying a flask of water to fill your gallon jug.. I think your only problem is that you need to know your riding style and plan out your trips accordingly; maybe get a controller with a 3 speed switch and set the lowest setting to super low amps to avoid voltage sag to cut off your pack.
 
Ok, I thought it's a common problem, that's why I started the thread.

Regarding high range. Pretty often I drive distances of something like 40 km one way, and then I need to come back. Because many factors influence the range of the bike (eg. peddling force, wind, SOC before the trip), it may happen that at the end of a trip the battery goes empty. My alarm gives me 2 warnings: one at ca. 15 km to go, and the critical warning when there is only 1-2 km range. I have 3 speed setting in the controller + additional 30 km/h limiter. I use it.

But still a "blackout" may happen and would be great to hold a remedy in backpack.

I am thinking of using two step-up converters from aliexpress that boost up to 35V and two small packs (1S or 3S each, depending if I cen get 4V converted to 35V in such a <2$ device) and go serial with these converters. In a worst case I would need these two converters + six 18650 cells, which will be roughly of a size of a laptop battery.
 
It could work, if you had a set of series connected cells..

For example,, suppose you have a 13s battery, charges to 54.6v. That's full. As you are near the end, you are down around 42v. A set of 13s cells full would be 54v or so. connect that pack to the charger port, and you'd have a charge flowing.

But its still not a good idea. Why? Because you will be hammering your 12 extra cells. Connecting 42v to 54v will cause a rush of current, but unlike your charger, no current limit on it. It will flow fast, hurting the cells, maybe fast enough to hurt your bms.

you can just connect that extra 13s pack when you start the ride. Connect to the main in parallel, but before the bms. You'd need a special plug for that. When the bms cuts off, it will also cut off your extender pack.

To charge, it's a naked pack. you can just leave it connected, but to balance it, it would have to be connected to the bms at the balance wires too.

Or,, connect it after the bms. Likely easier to do this. Always connect only when both packs are full. After the ride, charge and balance separately, as a naked pack.


There is a simpler solution. Ride slower the whole ride when you know you need that much range. It's pretty astonishing how much farther you go at 18 mph, vs 20 mph. And a 2mp slowdown is not that intolerable.

But in the end,, nothing works like a bigger battery. ASAP, get enough cells to add 1 p to your pack, and get 2 ah more range. 100watt hours is 4 miles more range, if you ride at 25 wh/mile. Adding 4 cells, not enough wh total to extend your range enough to be worth it.
 
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