how do you tell when lifo4 battery is low on charge

I also have some pingping batteries. Yeah it's amazing how far you can ride.

I have a cycle analyst hooked up to the bike. It's an excellent dashboard that tells you speed, amp-hour usage statistics, current voltage/amps and more. I highly highly highly recommend getting one.

Poke "cycle analyst" into your friendly yahoogle and it'll come up (ebikes.ca IIRC). It won't tell you precisely when the pack is gonna run out, but it'll give you more data with which to feed your need to know more.

Wtih the exact same set up, I get 23 miles of range on a " standard ride" I use for a range test. The ride is done full throttle with brisk but not exhausting pedaling. I weigh 180 pounds and the ride is done on a day with little wind, and the route goes slightly uphill and then down again with a total vertical of 500'.

Out in the real world, I have had range as low as 15 miles, climbing 1000 feet of vertical with 30 mph headwind, to a personal best of 28 miles done riding slower, but with some headwind.

I'd say unless the weather or a big hill is going against you, you can pretty much count on 20 miles at full speed. If you don't have a cyclometer, get one. Even if it is miscalibrated and giving wrong data, you can still use it to know where you stand on expected range left.

Of course the best way, is to have a watts up or a cycleanalyst, and measure the real capacity of your battery and then how much you have used, and a quick subtraction telly you if you are ok.

I find that charge time, and the odometer is all I really need. Even if I have no idea if my battery is full or not, I do know that 4 hours of charge time on the charger at work has always got me home, and so forth. With that 4 amp charger, I know that 6 hours will do a full charge, so you can figure out how far a given charge time will take you, plus the unknown amount that was still in the battery when plugged in. Pretty soon you will get the general idea what it takes to get to various places and planning a trip gets easier.

I have 2 Ping 36 volt 20AMP HR batteries on my bike.
I also get about 23 miles per battery pack. I have a
Watts Up meter mounted on my bike. The voltage stays
around 38 to 39 for 20 miles. Then when you notice a
voltage drop under 38 you will start shutting down. My
guess is that is the time to recharge. What I do is run
15 miles on battery 1 then 15 on battery 2 then back to
battery 1 keeping in mind I should be 10 miles or less from home. I love my battery system, I can range out on a ride farther than I ever dreamed.

Riding slow enough can get you home too, with less effort than pushing a motor that is dead. My personal best so far is 39 miles on a 36v 20 ah pingbattery and the bl36 motor. That ride was mostly flat, breezy but not windy, and 15 mph. Riding at 10 mph, 50 miles might be possible, so if you think you are that low, just ride home slower.