Compression Ignition Addict
Join Date: Dec 2007
Thanked 422 Times in 410 Posts
Mentioned: 32 Post(s)
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The GPs have a significant battery load (up to @150A) and the reason we have Two batteries. So, YES, 8.9v is way too low and good batteries will run the plugs AND crank the engine over at the same time maintaining 10+v.
If they're over 5yrs old and doing this I would just replace them Both (should always be done in pairs). If they're newer than that then I would have them Load Tested, at a High Rate (not one of those dinky 100A testers the parts stores use, out Big batteries need to be loaded to @300A to properly test them).
So charge them up fully and have them individually tested at some place with the right tester or maybe consider Harbor Freights 500A tester for @$40 w/coupon...