Battery Issues / Charging Issues (Part 3)
Note: It is important to note that all batteries in a battery bank should always be replaced with identical batteries of the same age. If just one defective battery in a group is replaced, the other older batteries will draw down the good one and each will charge at different rates causing a number of battery related issues. These include greatly shortening the life of the new battery, defeating the purpose in replacing a single one. Thus if one battery is defective then this requires replacement of the complete set of batteries, (i.e. All Coach batteries or All Chassis batteries).
Case Study 1
This coach had Absorbed Glass Mat (AGM) maintenance free batteries all of which were three years old. Periodically after storing the coach for a few weeks, the Chassis battery would be discharged to the point that the engine would not start, even though the Chassis battery disconnect switch had been turned off during storage. (It is worth noting that not all chassis battery functions are turned off by the Chassis Battery Disconnect, however the amount of parasitic power taken off was not considered sufficient to deplete the battery to a level where it would not have sufficient power to start the engine). At other times, leaving the coach in storage for the same or longer periods of time, the engine would fire up. Careful and thorough checking of the batteries, charging systems, battery connections, including the ground connections and measuring the parasitic current draw did not reveal any issues. This "No Start Situation" repeated itself sporadically during storage however the vast majority of the time, the coach was able to be used without concern about whether it would start.
After stopping to fuel the coach, it would not start and depressing the Boost Switch did not add the expected starting current from the Coach Batteries. Measuring the voltage of the starting batteries showed that they were low. The Coach batteries appeared to be fully charged. By temporarily jumping the Relay (often called a 'Big Boy Relay' because of its large size and capacity), full power was available to boost the chassis battery and the engine started.
Ground Cable was Broken Away from the Battery
During this inspection, it was discovered that a bolt molded into the lead negative ("-" or ground) terminal had broken loose and the cables connected together to that bolt were not always connected to the battery. Many Deep Cycle batteries have a combination terminal that includes a threaded bolt to place lugged cables on and a conventional battery post. Since this bank of batteries is connected in parallel, the system still had 12 Volts, however when the connection was not making contact with the battery, the coach system was being powered by only one half of the coach battery bank. The broken negative battery cable connection likely contributed to erratic control system readings to which the charging system would have reacted, with the outcome being that the batteries were not being fully charged. The solution was to place a conventional battery clamp onto the battery post and connect all the cables to that clamp and ignore the broken bolt, removing it completely.
Battery Boost System Failure
After the inspection revealed that the Boost Switch was not operating, troubleshooting revealed that the fuse powering the Boost Switch was blown. It was rated at 5 amps, which a quick calculation determined that the amperage was insufficient to activate the "Big Boy Relay". After making detailed inquiries and reviewing Service Bulletins it was determined that this fuse should be replaced with one with a 7.5 amp rating. That somewhat insignificant fuse was critical to the proper operation not only of the Boost Switch but more importantly the control system's link to the two sets of batteries: the "Big Boy Relay". This explains why the chassis batteries were so low after being boosted that they would not start the coach, since they were not being charged while driving. That, in conjunction with the broken battery connection, contributed to the failure of the coach battery system to be fully charged when "plugged in". After replacing the fuse, charging resumed and starting the coach multiple times afterward indicated that the problem was resolved. All Battery systems appeared to be operating correctly and battery charging was occurring from the engine alternator and inverter/charger.
Dead Batteries Again!
With these repairs, the owner embarked on a 500 mile (800 km) trip over two days that should have fully charged all batteries. After stopping for fuel, once again the coach would not start. The dreaded return of the intermittent "dead" chassis batteries. Thankfully with the new fuse on the boost switch it could be used to link the coach battery to start. Further investigation and examination of all Battery, starter and ground connections revealed that there was no other obvious faults. The Chassis Batteries were suspected of being the source of the problem. AGM batteries have one characteristic when discharged below the recommended 50% charge: that is that they may not accept a bulk charge from any power source. They need to be slow charged up to the point of being closer to fully charged. With this knowledge, a 10 amp battery charger was connected and left to slowly charge the batteries. When they indicated Full Charge, the batteries were disconnected from the Coach charging systems (and each other) checked for voltage (which indicated both were fully charged) and left overnight). The next morning, one battery still showed it was fully charged, while the other was 10.5V (dead).
It appeared that this battery was defective, since at times it would show that it was fully charge and now it was dead. That battery either had a construction defect or was sulfated with the other battery testing OK. Since the batteries are connected together in parallel the good battery was being drawn down by the bad battery and over time as the bad battery intermittently failed, the coach was unable to be started on one battery. Replacement of both batteries solved the problem. Interestingly the engine seemed to turn over quicker than it ever did. We surmised, given the erratic history of incidences, it is likely that a battery with a construction defect was installed when the batteries were replaced three years earlier.