Introduction & a question

[yuhr]

1. I am new here, just registered a minute ago. Greetings!
I am not an engeenier of any sort; used to make portable radios at my younger age- transistors just appeared!
2. I recently added "electic bike conversion kit" to my regular mountain bike. Usually I don't take my bike very far; this kit nominal range is 19-34 miles (contingent on rider's weight, terrain, etc.). My motor is Direct Drive, powered by 36V 9 Ah battery - actually its pack consists of 3 12V batteries in series.
Strangely, there is no battery discharge LED indicator, which is a standard feature of most kits. I have a digital multimeter but it would be esthaetically unpleasant, odd looking and quite awkward to mount it on a handle bar. I wonder: could I build a simple LED indicator to measure battery voltage, so I'd know in advance that battery is getting low and it's time to recharge or make a U-turn and go home? If the answer is yes, where can I find diagrams, etc.?

 

[BrownOut]

Hi yuhr and welcome.

I'm not the most versed on battery discharge,but typically batteries have a flat voltage discharge curve until there is very little charge left, then the voltage falls off. I fear any simple voltage measurement will leave you far away from home with a dead battery. Prehaps you are better off with a circuit that monitors battery current and caculates the charge left in the battery.

There are other who have better knowledge about battery discharge cycles than I. I'm sure someone will come up with something grand.

 

[ericgibbs]

hi and Welcome,
You may find this link interesting.
https://www.electro-tech-online.com...le-entry-11-building-bike-rack-continued.html.

A LM3914 makes a simple battery status LED display.

 

[yuhr]

Thank you, guys. I think that LM3914 could be a good starting point. All projects so far are for 12V car battery and I wonder what kind of modifications I'll have to make for my 36V battery. Fully charged battery is slightly over 39V and is around 35V when it's nearly discharged (the wheel can still spin but only without load, i. e. without me ). I will continue to dig online - I still have time, as my kit is still under the warranty and I do not want to do anything to void it.
BrownOut, what you said is true for alkaline and, perhaps, other solid state batteries. AFAIK, the best indicator of acid-lead battery status is its electrolyte density (American Navy did actual measurement of electrolyte SG on its diesel submarines, I don't know how it's done nowadays). Monitoring battery voltage is the closest proxy and is widely used, at lest for Sealed Acid Lead (SLA) batteries and open car batteries.

 

[BrownOut]

Look at some typical discharge curves for sealed lead-acid batteries. They are flatter than alkaline. Just know your batteries and pick your limits carfully.

 

[ronv]

I used a 3914 for my golf cart. It worked pretty good. I found some green, yellow, red led bars. It was 48 volt so you need one of those little 5 volt switching regulators to build a supply for the 3914. I can't remember for sure, but I think the battery voltage (at rest) dropped about .12 volts for every 10% of discharge with fully charged being 2.12 volts per cell.

 

[Dragon Tamer]

You could try an ICL7665 or an LM393 with a voltage reference. I would go with the ICL7665 simply because it is a much simpler design and the equations to go with it are clearly defined. The catch is that no matter what chip you chose you will need to lower the supply voltage to them by a fair bit. No need to go fancy a simple emitter follower should work just fine. In both cases even though the chip is supplied by a lower voltage than what it is monitoring, you can still monitor very large voltages with the sense pins.