What is the voltage sag on your ebike?

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VelluK   100 mW

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What is the voltage sag on your ebike?

Post by VelluK » May 12 2018 4:15pm

What is the voltage sag on your ebike when you have turned the full throttle from 0 mph / kmh?

I'm wondering the voltage sag on the other thread and I'm interested to know what kind of voltage sag you get on your ebike. What is the normal and what is not.

Thanks!
Stealth ebike, hub motor MXUS V3 3K Turbo, controller Kelly KLS7230S, batteries Sony VTC5 20s6p (84V 15.6Ah), rear wheel moto 21"

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Willow   10 kW

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Re: What is the voltage sag on your ebike?

Post by Willow » May 12 2018 6:42pm

Factors that influence sag are: Discharge rate of the cells, Capacity of the pack, Amp draw, Temperature, Type of cell chemistry, Age of cells/state of health.

...so everyone will have a different answer for you.

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amberwolf   100 GW

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Re: What is the voltage sag on your ebike?

Post by amberwolf » May 12 2018 11:39pm

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Last edited by amberwolf on May 19 2018 1:01pm, edited 1 time in total.

tolkaNo   10 W

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Re: What is the voltage sag on your ebike?

Post by tolkaNo » May 13 2018 2:39am

Im still building my bike but with 13s5p sanyos (48v, 17.5ah) i sagged 1v when i was spinning a 1000w hub motor unloaded on a 35 amp peak controller

I havent ridden the bike yet so thats my best answer so far

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dogman dan   100 GW

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Re: What is the voltage sag on your ebike?

Post by dogman dan » May 13 2018 6:54am

I'm big on rule of thumbs for things. I have one for range at typical e bike speeds, and another for sag. I think sag over 6v is bad.

If full throttle from a stop gets you only 2v of sag, that is outstanding. This will likely mean less than 1 v sag at cruise. Its typical for a bike using RC batteries of at least 10 ah size, and 30c (claimed) c rate. To get this, you are actually pulling 5c or less usually, with ordinary cheap hobby king lipos. So a 1000w setup, doesn't hardly strain this type battery at all. Up the wattage ante enough though, and you can be back to high sag on takeoff. That is where you are, that sag is costing you acceleration. A larger pack will sag less, but it may still not get your wheel up. A more poorly balanced bike will get the wheel up at 500w though. So you might look a bit at putting that additional battery farther back.

Other types of batteries might be 3c claimed, or at least not much more. This is typical for e bikes in the sub 1000w category.

In this case, more sag is definitely typical. IMO, if the sag is more than 6v, then you tend to have a battery that gets very noticeably hot, not just toasty warm by end of ride, and this is not acceptable to me. It might be ok to the industry, who's only goal is to make that sale. To make that sale, they undersize the expensive part, the battery. They know it will last the warranty period, but not really last. So they will gleefully sell you a system with 8v sag or more. Bear in mind, cruise sag may be ok, but start up sag not.

4 or 5v sag is ok, so size your battery so that your motor system does not produce much more than that, or, ride such that you only see that 4v sag. Feather the throttle on take off, then give it full power once moving 10 mph. This will help your battery keep its cool, and last a lot longer. Ideally of course, you pack enough battery to get 4v sag or less no matter what. But it may not be all that practical to carry 30 ah to get this. One option is to parallel your low c rate 18650 cell pack with some higher c rate lipo, when you will need to pull the bigger amps spikes. Like mountain riding. Then you get the best of both worlds, without going to a 1500 watt hour battery.

Antenor   100 W

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Re: What is the voltage sag on your ebike?

Post by Antenor » May 13 2018 10:13am

I can parallel a 48v 14.5A ga cels with a 48v 10A 25r cells?
I was convinced that have to be the same cell type...

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Willow   10 kW

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Re: What is the voltage sag on your ebike?

Post by Willow » May 13 2018 4:53pm

Antenor wrote:
May 13 2018 10:13am
I can parallel a 48v 14.5A ga cels with a 48v 10A 25r cells?
I was convinced that have to be the same cell type...
Don't do it...

footbaldd   100 mW

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Re: What is the voltage sag on your ebike?

Post by footbaldd » May 13 2018 6:31pm

I think it would be beneficial to talk about sag in terms of volts per cell instead of overall sag. 6 volts of sag on a 48v battery is not the same as on a 100 volt battery. At 90 amps I was seeing about .22-3Vpc sag on my 21s12p pack using LG HG2 cells. I need to look at it more closely, but I think that is about what I was seeing. It also varies with state of charge. They say they are rated for 20A continuous; however, if you look at the voltage vs load, I think 15A is more realistic. For my pack that would mean I don't want to pull more than 180A.

I have dual layer nickel strips on the series connections (8mm x 0.2mm) with large copper plates on the ends, and then 6awg weld cable for the rest. There is a lot more testing I need to do, but that that should give you a ballpark.

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Re: What is the voltage sag on your ebike?

Post by Matador » May 13 2018 11:39pm

worst thing I had was 9-10V voltage sag using 14S8P old laptop cells with 1500W draw BBSHD at 30 amp. The cell were really hot.... 70°C !

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