The theory on a double charger is it feeds both batteries individually, so for example they each receive a 10 amp charge, and then if you have a smart charger the maintenance cycle starts. If you have a single charger (or a double wired as a single) and feed one battery linked to a second then the first battery is charged and then the second battery.
Depending on how long the charger is on you may charge the first battery but not full charge the second.
DWH is resident "Electron Guy" on the forum, I pay close attention to what he writes, so I'm interested in what he has to say about my double charger theory.
(Martyn, I know you already know most (if not all) of this, so I'm not lecturing you in particular here - I'm just spouting off for whoever happens to stumble in and listen.
)
Current will flow from the supply into the bus, then from the bus into the loads. In this case, the supply is the charger and the loads are the batteries.
Each battery will absorb however much current can overcome its internal resistance. The nearly full battery will have a higher resistance than the "somewhat" discharged battery, and will absorb current at a lower rate. I.e., more current will flow into the lower battery.
Once both batteries reach whatever voltage the charger is holding the bus at, then they won't absorb any more - the voltage is equal at the charger and at each battery.
So they won't be charging equally - but they'll both reach the finish line (bus voltage) at about the same time.
A very deeply discharged (dead) battery can have a resistance even higher than a nearly full battery, so in that case, the higher battery would absorb more than the lower (dead) battery, until the lower battery had absorbed enough to decrease its resistance, and then it would be absorbing more than the higher.
The higher battery would absorb more than the lower battery at first, but once the lower battery's voltage came up to where its resistance dropped, then current could flow from both the charger AND the higher battery into the bus, and then into the lower battery - until the lower battery gets up to where it doesn't absorb so much.
For instance, if the lower battery could absorb 30a but the charger was only supplying 20a, then 10a would flow from the higher battery and 20a from the charger until the lower battery was full enough to absorb less than 20a, then the current would all come from the charger which would then bring them both back up to the set voltage.
During the time in which that situation is occurring, there would be two supplies and one load on the bus.
That's with a constant
voltage type charger. No worries. Even if one battery has longer wire it won't much matter, since it will still eventually reach bus voltage.
With a constant
current type charger there is a potential problem.
Say the charger is forcing 20a to flow. One battery is really dead and has a very high resistance. The entire current flow will end up getting forced into the nearly full battery. The charger is looking to see what the bus voltage is so it knows when to back off, but the bus voltage is being held down by the dead battery and so the charger just keeps pushing and overcharges the nearly full battery.
This can also happen when you have a battery bank of mismatched batteries - different size, or different type or different age. It can also happen if you have a bank of matched batteries, but one fails. In that case, you start a chain reaction, where the failed battery causes the charger to overcharge a good battery, and it eventually fails, and so on until the whole bank is toast.
And then there is the "multi-stage charger, charging two different batteries" issue...
A good 2 or 3 stage charger does both types of charging - constant current and constant voltage. Generally, "bulk" stage is constant current and "absorb" and "float" stages are constant voltage.
So say you have a low battery and a high battery tied together and are charging with a 3-stage charger. Bulk stage is generally not a problem, as the charger will force its full output - say 20a - into the bus, and the current will get divided up based on the resistance of each battery. The charger will do bulk stage until the bus voltage reaches a certain point - say 14.4v.
Again, they won't charge at the same rate, but they'll get to the finish line at about the same time.
However...even though they've both reached the same voltage...the low battery isn't actually full yet, but the high battery is. The low battery still needs to have its chemistry stirred up a while longer to have absorbed all it can and be completely charged.
So now the charger sees that the bus voltage is at 14.4v, so it switches to constant voltage mode for the absorb stage, and holds the bus voltage at say 14.2v. It will stay in absorb until the current flow drops below say 1a.
The lower battery is absorbing current at a rate greater than 1a, so the charger keeps the bus voltage at 14.2v - which is perfect for the lower battery, but is too high for the higher battery; The higher battery is full and shouldn't be held at that voltage for a long time.
By the time the lower battery is full and the current flow drops below 1a and the charger drops to float stage and lowers the voltage to say 13.6v - the higher battery has been held at 14.2v for too long and has been overcharged.
The same problem can occur if you are charging just one battery (or bank) with a 3-stage charger while there are loads running from the battery. If the loads draw more than 1a, then the charger thinks the battery is still absorbing (even though it's full) and keeps holding the bus at 14.2v and overcharges the battery.
In either case, it's recommended to use a 2-stage charger instead of a 3-stage, so that the charger drops straight to float after bulk - there is no absorb stage so the charger can't be fooled into staying in absorb.
Some chargers get around the problem by setting a timer on the absorb stage, so that it will drop to float either when the current flow drops to under 1a, or when the timer runs out, whichever comes first. But that's a little bit imprecise. It could still end up overcharging the higher battery - though not as badly - or it could end up not getting a complete absorb done on the lower battery.
An Iota charger is 2-stage by default, and 3-stage with a timer on the absorb stage with the IQ/4 module. The Samlex SEC-1215a that I plan to install in my truck has a dip switch to set it in either 2-stage or 3-stage, but has no timer on the absorb stage - so when running it in 3-stage, it should only be charging one battery and with no loads on that battery.
That's fine - I plan to run it in 2-stage mode anyway, since I'll be using it to both top off my engine battery and recharge my house battery and will often have loads running from the house battery at the same time.
Okay...now back to Martyn's question...
Two batteries, not tied into a permanent bank.
Two chargers (say 10a ea.).
A) Is it better to tie the batteries and tie the chargers and supply 20a to both batteries?
B) Is it better to split them and give each battery its own 10a charger?
C) It depends.
The answer is A.
Unless the answer is C.
Why? Efficiency.
If one battery gets full in say 1 hour, and the other will take 4 hours, then for 3 hours, your 20a worth of chargers will only be putting out 10a.
Let's say you need to replenish 20 amp*hours into one battery, and 50ah into the other. You need to supply a total of 70ah to get them both charged. (I'm ignoring battery inefficiency calculations here.)
So you split them and each battery gets 10a. One battery will be full in 2 hours and then half the available charging current (one charger) will sit idle. The other will keep on keeping on for 5 hours to recharge the other battery.
Total time to charge both - 5 hours.
If you tie the batteries and chargers, then to supply the 70ah needed to recharge both batteries will take less time since you have the full 20a available throughout the process.
Total time to charge both - 3.5 hours.
(That's just an example to give the general idea - the actual charging times will probably be at least double what I just said because getting the last 10% into a battery takes a very long time.)
Unless the answer is C.
If each battery is rated to handle a max charging current of say 30a, and you've got two 30a
constant current chargers - then you wouldn't want to tie them and potentially force 60a into one battery.
But if they were
constant voltage chargers then it wouldn't matter since the battery will only absorb X current at Y voltage anyway. The rating of a constant voltage charger is simply the max amps it can supply - not the amps the battery will actually draw.
Hopefully I've sufficiently muddied the waters for one day.
