12v battery. Should voltage drop while cranking?

[crooozer]

Hi all,

I have a riding mower that has just had the starter motor brushes replaced and the battery went bad at pretty well the same time (brush assy went bad because I tightened the terminal too much, tearing the wires off it while cleaning the contacts) Anyhow, I tried to start the mower off the Landrcuiser, battery in that measures 12.7 volts sitting but the starter turns too slow to crank the engine while I try.. It measures 10.2v while cranking the mower engine.

Then, I turned the Landcruiser on so it was charging, cranked the mower off of that and it started no problems.

Why is this, could something be wrong with the mower electric circuit thus needing more volts to start? The battery measured 15v while charging.

Thank you.

 

[Bret]

A couple variables to consider. Jumper cable connections can sometimes make starting a vehicle via cables difficult. Check your connections at the battery, a corroded connector can drop the voltage similar to that. With a good charged battery and clean connections I wouldn't expect the voltage to drop that far. While starting the 40 maybe, depending on the capacity of the battery.


Assuming your mower is a small riding lawn tractor anyway...

 

[brew8]

The voltage across a battery under load is dependent on it's internal resistance. When It's running there are two voltage sources in Parallel. A lower voltage is common under high load because the current loop may be dropping more voltage on the load because of how a current loop works under load. you may also have a weak cell if it is a 2 volt reduction under a lighter load as you described.

 

[peesalot]

all depends on the cable connections while jumping.

 

[brew8]

Your right

 

[crooozer]

Thanks guys. It turns out the top poster was absolutely correct. Nothing compares to a good strong battery terminal bolted on. I removed my battery, screwed it on the mower and she turned over no problems!

I just bought a new battery and also a float charger. Apparently these make your batteries last in many cases years longer.

I was speaking to a guy who has them for all his batteries and he gets 5-7 years out of each one now!