|If a battery repeatedly only charges to 80% then you may have a bad charging cord set. The battery needs voltage higher than what currently is in it in order for more 'juice' to be pushed into it. Since the actual voltage rises as it is charged, the voltage needs to be greater to complete the charge than it needs to be before it reaches the 80% level. A 6 cell battery is nominally 7.2 volts, and even 10 volts can charge it when it is low, but when it reaches 80% it is probably closer to 11.5 or 12 volts so you would actually need at least 13 volts to continue charging and possibly slightly higher to reach 100%. I have not done the exact math, but these figures are pretty close to what actually happens during the charge (you get the idea) so if the charging cord is bad and not delivering the voltage, then you are not going to get more. If you know someone with a similar system, you could borrow their cord to prove this.|
Another possibility is that you have always charged the batteries before they were low. If this were going on for long enough, the battery may not be able to accept a full charge. Sometimes running it down until it nearly shuts down and fully charging it 2 or 3 times may improve things in this case.
You have to be a little bit crazy to keep you from going insane.