How to tell when Lead Acid Batteries are dead?


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Hey, I have an Emmo knight and I am able to get about 16-18km on a full charge on speed mode 2 without going too hard. I opened up all the fairings and tested all the batteries and they all have over 13V, I’m about to pull the trigger and buy new batteries but I’m still not sure how to test my old ones to make sure it is batteries. May someone suggest a way?
Auto parts shops, and presumably scooter shops have load testers for lead acid batteries. Probably battery shops too.
I use an analog voltmeter on the starter terminal. If the battery has a voltage of 12.8 on the terminal no load, but goes below 9.5 when the starter is turned on, then the battery is sulphated.
No starter? then build resistor to load the battery equivalent to the expected load on the battery. I checked my bike battery with a load of 5 amps at 48 v. (10 ohms >250 W). Pretty light load for a battery rated at 50 amps draw, but the battery failed, collapsing from ~52 to 11 volts.
Reason not a DVM, they average over 2 to 4 seconds. analog voltmeters are instantaneous.
The easiest way is to see how it behaves under load.

I presume these are AGM, not flooded? Fully charged AGM should have 12.8-12.9V open-circuit, measured at least 12 hours after you've disconnected it from the charger. Not immediately after charging. Since voltage in this measurement doesn't change quickly, a regular DVM with 2-3 decimal points will work fine.

Then apply some load - say, 10A per 100AH battery - and see if the voltage drops below acceptable level. If it drops below 11.5V-11.8 or if the motor is struggling or the bulb goes dim, it's time to replace.

If you don't see motor struggling with your usual mode of riding and the range is good enough for you, there is no need to replace immediately, just keep an eye on it .
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Thank all of you guys, I ended up just going out and buying a battery load tester and they all tested weak, I will be buying new batteries.