Batteries shows fully charged. 24 volt shows 27 volts fully charged but shows RED light when install in mower.

Replace the exhausted battery.

Hi Linda.
not sure if this will help you out,i found this on the internet.
Battery Charger Troubleshooter
When there is a battery failure, there is a possibility it was caused by a bad battery charger. When the charger is only plugged into the wall, it should show a green light. If there is no light showing, confirm that the outlet is good. If the outlet is good, it is very possible that the issue is with the battery charger, not the battery.
If the charger is plugged into the battery only, it should show a green light. (If you have purchased a new charger within the last year, you will not get a green light if the charger is only plugged into the battery. You will get a green light however if the wall charger is just plugged into the wall and not plugged into the wiring harness) If it does not show a green light, it is likely that the charger is not making contact with the battery. The most likely reason for this is a faulty fuse on the wiring harness. This fuse is a 2-amp automotive fuse, and it should be replaced with a 2 or 3-amp fuse available anywhere that sells auto parts. The fuse can blow out if there is a short in the charging system, or if the plug-in end of the wiring harness comes in contact with a battery terminal.
When the charger is plugged into both an electrical outlet and the battery, there should be a red light showing on the charger. A red light showing indicates that the charger is in contact with the battery and is actively charging the battery. Once the battery has reached a voltage level that is considered "fully charged" the red light changes to green. If after 24 hours the light has not changed to green there is no cause for alarm! Remember that when the light is red, the battery is being charged, and you will be able to use your system. After 24 hours or so, your battery is as charged as it is going to get, and if the light is still showing red you should disconnect the charger and go fishing. Replacement chargers and parts can be purchased directly from MarCum's website.
If you are really concerned about your battery, consider getting a basic volt meter to check the voltage of your battery. They can be bought for less than 20 dollars and will show you exactly what the voltage level of your battery is. If after 24 hours of charging your battery should have at least 12 volts (ideally 13-14 volts) and as long as it is over 12 volts it will work. If it is not at or above 12 volts you should consider replacing the battery. If your unit starts to malfunction while you are fishing, it would not come as a surprise that the voltage has dropped down to 11 volts or lower.
If the charger is plugged into the wall and battery, and you get a blinking light on the charger, it is very likely that the battery is faulty and should be replaced.
Batteries are an expendable item, and must be replaced periodically. Any store that sells ice fishing electronics will stock suitable batteries. You can also buy them at a specialty battery store, or by ordering one from our website.The batteries that you should use are the "Sealed Lead Acid" variety, they are 12 volts, and range from 7 to 9 amps. The more amps the battery has the longer it will run on a full charge. Your MarCum unit can be powered off of any battery that is 12 volts, even a large automotive or deep cycle batteries will work.
or go to this link and have a read up.

battery charger is blinking geen Google Search

If the batteries were discharged completely, the battery charger isn't strong enough to start the charging process. Take the batteries out and charge each one separately with an automotive type battery charger that is NOT automatic. This may take a day to recover this way. When the battery shows over 10 volts with a dc voltmeter you can re install them in the chair to complete the charge with the 24 volt charger. If the batteries have any amount of age past 2 years, it's time for new batteries.

An unloaded reading of 10 Volts on a 12 Volt car battery is considered dead. There are 6 cells in a typical lead acid (car) battery - each provides 2 volts. They are internally connected in series to provide 12 volts on the + and - posts. If one is cell shorted out - you'd see 10 volts if fully charged. Trickle charge the battery for 24 hours and measure voltage again. If 12 volts, bring it to be load tested. that will tell you the true state of the battery.

Check the switch for current flow with and without the key. Then make sure the ground switch is disengaging when you pull the handle.Possible stretched cable.

Check your battery wiring on the Jet the wires from the circuit breaker need to connect the positive of the front battery to the negative of the rear battery and the other wires connected to the positive of the rear battery and negative of the front battery putting the batteries in series to generate 24+ volts. If you have a voltmeter before you check the wiring set it to 200 volts dc and plug the leads into the 2 outter holes of the charger port located under the front of the joystick. If your batteries are wired correctly you should be reading 27+ volts without the charger plugged in and your batteries fully charged. If you are only reading 12 volts then the wiring is wrong and needs to be fixed. Leaving the voltmeter in the charger port plug in your charger and you should be reading 27 -30+ volts at the charger port anything less than 24 volts indicates your charger is not charging Hope this helps

John the red flashing light indicates a low battery When you charge the battery your charger should be showing a red light for power and a yellow light. When the batteries are fully charged you should show a red and green light on the charger. Now if you have a voltmeter plug the leads into the two outter holes of the charger port located under the front of the joystick.Without the charger plugged in you will be reading the battery voltage. Fully charged it should read 27-30 volts if you are only reading 20-24 volts then the batteries are low. If you have an onboard charger plug it in and you should read the charger voltage of 29-32 + volts If you aren't getting that voltage then you have a bad charger if you are getting the charger voltage then you have bad batteries and they need to be replaced Hope this helps you

flobo I assume that its the on-board charger and that it is the gray charger with one light. This charger light will only come on when the chair is fully charged. Now if you have left it plugged in all night and still have no light then I would think you have a bad charger. If you have a digital voltmeter you can check the battery voltage throught the external charger port under the joystick. Set it on 200 volts and insert each probe into the outter pin holes and you should be able to read the voltage. Without the charger plugged in you will be reading the total battery voltage 24-27 volts with the charger plugged in 27-30 volts also check the meter with the charger plugged it should show something above zero and gradually decrease as the batteries become fully charged. Hope this helps

When batteries are only used seasonally, and they are stored, they can become damaged from weather or from storing them when they are not fully charged. In general, batteries used seasonally will have a shorter lifespan, so it sounds like you likely need a new battery. Check out http://www.batteryinternational.com/electric-lawn-maintenance-batteries.html for a replacement, and be sure to charge it fully before storing.