Posts: 2754
Location: Mauston, Wisconsin | Without getting into battery chemistry and calculus. Urban legends abound about batteries.
1. When a newer battery is connected in parallel with an older battery, the older battery will not shorten the life of the newer battery. This was the fundamental question posed in this thread.
Therefore my first post-> "there is no sin in connecting batteries of different ages in parallel. Just don't expect the older battery to last as long as the new one."
2. If adding a new battery to a series string (24V system) the older battery can shorten the liife of the new battery, ONLY IF they are charged as a series string.
If they are charged individually (dual bank charger, etc.) there is no danger to the newer battery.
3. Using an A-B switch on two paralleled batteries is good, if you understand the advantages of the A-B switch.
A. It can save the day if you have dual batteries for your engine starting batteries. You can run off one battery and keep the other in reserve.
B. It can allow you to charge each battery individually if you don't have a dual bank charger.
C. If using A-B switch for your trolling motor always run it in the A+B position (the batteries are then connected in paralel). This will give you ~3 times the runtime as using a single battery. Whereas Juno's sugestion will only get you 2 x the runtime, i.e., because you are discharging them individually at a higher load current.
Ok- trying to simplify this. Some folks think about batteries like gas tanks, i.e., that they hold a specific amount of fuel/power. That is true if they are used individually. However, if I start paralleling them-> connecting the tanks together some strange things start happening. They no longer hold the same volume of fuel to power the same engine. The volume of available fuel increases. Yet the physical volume of the tanks is the same!
F. ex. If your trolling motor draws 30 amps at full power, and if you connect it to the batteries individually each battery can only discharge 30 amps for x time. However, there is this funny little rule about lead-acid battery electrochemistry where: If you reduce the load current by 50% you will get 3 times the original runtime. By connecting another battery in parallel we effectively reduce the load current to 50% on each battery in comparison to an individual battery. This means each battery is now supplying 15 amps and 2 x 15 = 30A.
Lets say my existing battery gives me ~ 3 hours at full power. If I connect another same size battery in parallel, I will get 9 hours. Whereas if I discharge them individually I will only get 6 hours (3+3=6).
I hope this helps.
Al |
