LEAD Battery voltage drop

[Burgerman]

REPEATED FROM A DIFFERENT THREAD. Because its important, and:

- Why lead batteries are never going to work right in off road chairs.
- why high speed motor packages are a bad plan unless you are light.
- why you NEED odyssey batteries over high resistance alternatives.
- why you need the biggest and best batteries you can get!

My fancy new Multimeter does graphing, trends, logging etc. Very cool.
But also does peak reading. Which shows the following.

So I connect it to my battery. Why?
Because I go on and on about battery resistance and how shit lead batteries are!

This shows how crap even Odyssey batteries are under load. And these have HALF the resistance of MK gel batteries. So are twice as good under load. Expect to see much more voltage drop from MK.

I leave my desk, drive around the house, and do a wheelie. Then I look at the meter.

And it reads:

-CURRENT VOLTS (26v, fully charged)
-HIGHEST VOLTS (regeneration voltage as I slow down 27.5 volts!)
-AVERAGE VOLTS (25 Volts)
-LOWEST VOLTS (18 Volts - when I turned hard or wheelied)

Remember these are FULLY CHARGED. First 100 feet after getting up!
And remember that Odyssey is MUCH better here than all other batteries. They can start a truck in Siberia.

The controllers protection circuitry starts cutting back power (you get less torque) at 18 volts. So it will not go lower than this. You just lose power.

Even so, I see an 8 to 9.5 volt difference between loaded up (turn/accelerate) and stopped or slowing down. And this is with GOOD batteries that are FULLY CHARGED!

A 33+ percent voltage drop under load...

[Burgerman]

Added. Got it down to 16.5v in the street, by a few wheelies one after the other...

That's almost half the 26v battery voltage!

Lead batteries are partly SPRINGY - resistance.
And partly squashy like a bit of cheese - that's Peukert.

And both increase as they discharge. And as they age.

[robnnorthaustin]

Wow, That surprises me. My meters don't save readings but I constantly look at my voltage and amperage under load and haven't seen a lower voltage then 22.9v on my MK's. I do have a lower amperage draw since I use scooters. I am assuming that I since I have a lower amp draw and I don't see very short peaks that I am seeing a much better readings. I wonder if you would still see such extremes if you could set your meter not to record loads over a half second or so?

[Burgerman]

Well it can graph it too. The voltages drop for a fair time. At least a second or so. At the times when you need it the most.

Of course my chair is programmed to respond instantly. And that's exactly what the voltage does too!

[Burgerman]

Its set to record every 0.1 of a second. So it gives a real reading. If you set it to record at say 2 times per second you would miss much of what was going on. As you will on a normal multimeters display. But you want to see it! Half a second is a long time if you were expecting it to turn or wheelie!

Of course with typical grandma stock programming you would see nothing like this.

That translates to about 35mOhm total system "resistance" according to ohms law. But its not just DC resistance you are seeing. Its the surface charge affect where the plate surface and acid interface discharges fast, and the rest of the charge is INSIDE the plate and in the acid further from the plate. Give it time and this recovers. To recover fully takes many hours...

[ex-Gooserider]

Note that unless you get into either a scope or a fancy meter like BM's the standard DVM only does a few readings per minute, and often won't give ANY kind of useful reading if the input voltage isn't stable... (A good meter will even have a spec on how fast it takes a read, and will tell you that the results are not predictable / valid if the voltage changes faster than this.) I suspect that if you had a good recording scope you might even see lower peaks than BM is reporting...

A DVM is of questionable value on any kind of changing voltage source. Rapid responding meters like BM's or the so-called "analog bar" displays on some fancy meters are an effort to make up for this and it does help, but there is a reason why for some tasks a good tech will still drag out the old analog "swing meter" or grab an o-scope...

In general digital circuitry can do some wierd stuff when powering up / down or during state changes, which means that you really need to be careful to lock it out when power-cycling and make sure you aren't trying to use an output that is changing state as an input or you WILL see odd, non-reproducible errors that are hard as heck to find...

ex-Gooserider