Using two chargers on one battery bank

[tgpt21]

When at anchor I use a suitcase generator to charge the batteries through the plumbed-in smart battery charger. This gives around 15 amps utilising 240 watts from the genny which is rated a 1000 watts. It does have a low power switch which we use but I still feel that we are burning fuel for little return.
I have bought a 30 amp smart-ish charger (reduced from £135 to £35 I cannot resist a bargain) for another purpose and I was thinking perhaps I could connect this second charger whilst the genny is on giving us the potential of 50 amps.

Would there be a problem in connecting both chargers at the same time? I cannot see any, as on occasions the shore power is on and the engine is running. But I may be missing something.

 

[PaulRainbow]

Not a good idea. Each charger will sense the voltage from the other charger and shut down, or something along those lines. Results will be unpredictable, but you will almost certainly not gain anything.


You could of course swap the 15a for the 30a

 

[rogerthebodger]

When at anchor I use a suitcase generator to charge the batteries through the plumbed-in smart battery charger. This gives around 15 amps utilising 240 watts from the genny which is rated a 1000 watts. It does have a low power switch which we use but I still feel that we are burning fuel for little return.
I have bought a 30 amp smart-ish charger (reduced from £135 to £35 I cannot resist a bargain) for another purpose and I was thinking perhaps I could connect this second charger whilst the genny is on giving us the potential of 50 amps.

Would there be a problem in connecting both chargers at the same time? I cannot see any, as on occasions the shore power is on and the engine is running. But I may be missing something.

I don't see an issue unlike PailRainbow.

Each charger will sense the battery voltage and providing this voltage is below both charger thresholds will still provide some current until the battery voltage rises then the charger with the lower threshold will stop charging until the battery reached the threshold of the second charger then both will be off

I have a setup with 3 banks of solar panels each bank with its own regulator. When I start my engine the alternator also provides charge. I also have a 50 Amp mains charger that when I have mains power I can also use.

I used to also have a wind generator that could also add charge when the wind blew without a problem.

Just don't expect to get all the 50 Amps from both chargers.

 

[PaulRainbow]

I also have solar panels, engine alternator and mains charger.

If i switch the mains charger on, the solar controller will see the output from the mains charger and it will shut down. I recently did some tests to prove this and the solar yield was between 10 and 30Wh per day !

 

[rogerthebodger]

I also have solar panels, engine alternator and mains charger.

If i switch the mains charger on, the solar controller will see the output from the mains charger and it will shut down. I recently did some tests to prove this and the solar yield was between 10 and 30Wh per day !

It does depend on the threshold voltages of the various sourced and the internal resistances of the chargers and the batteries.

I see this as when I arrive at my boat after being away for some time the battery voltage is quite high and the solar regulators are all off but as I use power and the battery voltage starts to decline the regulators start to supply current as the battery voltage comes below the turn on voltage of the particular voltage.

the main point is the voltage at the battery is not the max output of the charger is a balance between the charger internal resistance and the battery internal resistance unless you have a very big charger and a very small battery bank then the battery voltage will rise quite quickly maybe to a level above the threshold of the other charging devices.

I have 12 of 105Ah battery bank so it can take quite some time for the battery voltage to rise to above the solar regulator thresholds.

All depends on setup

 

[benchmark]

When at anchor I use a suitcase generator to charge the batteries through the plumbed-in smart battery charger. This gives around 15 amps utilising 240 watts from the genny which is rated a 1000 watts. It does have a low power switch which we use but I still feel that we are burning fuel for little return.
I have bought a 30 amp smart-ish charger (reduced from £135 to £35 I cannot resist a bargain) for another purpose and I was thinking perhaps I could connect this second charger whilst the genny is on giving us the potential of 50 amps.

Would there be a problem in connecting both chargers at the same time? I cannot see any, as on occasions the shore power is on and the engine is running. But I may be missing something.

I can already tell you now that it won’t work. Both smart chargers use microprocessors that constantly calculate the batteries state of charge, rate of charge,charging time left etc. and the presence of another charger will interfere with this process and cause suboptimal performance, maybe even malfunction.

If you doubt me, connect them to a testbench battery setup and see.

PS: this info should also be in the user manual of your new battery charger.

 

[bedouin]

Don't do it.

Firstly in almost all charging situations you are limited by the amount of current the batteries will absorb, not by the output of the alternator / charger. Secondly a decent smartcharger adjusts its output depending on the state of the battery - attach another charger and all that cleverness can't work.

The only way you could do it would be if you had 2 or more batteries that you could charge individually from different chargers (i.e. disconnect from each other)

 

[solent clown]

the only way to make that work would be to split the bank ( a manual cutoff switch would do it)

 

[matthewriches]

It is possible by using some power management arty farty stuff but best not worry about it. You may end up with each charging device "chattering" causing potential damage.

Can you not get a bigger charger?

 

[lw395]

It depends.
If you have two completely 'dumb' chargers, and a big, fairly flat battery bank, then you might find they both put in a good current.
This might be fine for a car that won't start and you've got a couple of £15 chargers.
With smart chargers, all sorts of things could happen. Just don't go there. Not all smart chargers are the same.

As Bedouin says, mostly it won't help anyway. Once the batteries are up to say 14.4V, the charging is no longer current limited, most of the time you will be in constant voltage charging.
If you are spending a lot of time wanting to charge at more than 30A, you are giving the batteries a pretty tough life.
I've heard of some people using one charger to actually charge batteries, and another to run loads like a fridge but that gets complex.

 

[Poecheng]

http://www.ybw.com/forums/showthread.php?321758-Can-I-parallel-battery-chargers

There seems to be alternative view on this earlier thread - including the response of CTEK

 

[William_H]

I think it is worth explaining that the rating of a battery charger is just a max rating. As said actual current into a battery depends on the state of charge of the battery and the size of the battery bank. (unless it is a very crude old fashioned charger which tend to function more like a constant current charger). So an amp meter will tell you exactly how much charge the battery is accepting at any given time. Usually high at first then reducing fairly quickly. if indeed you have a very big very low battery bank then your charger may be the current limiting factor. In which case another charger may well assist for a period until the battery comes up when current will fall so the additional charger becomes redundant.
note unfortunately the battery voltage rises under charge to a voltage somewhat more than the real voltage off charge (which might indicate charge %).
So I would suggest OP fit an amp meter into charger circuit of either charger and just observe what current the charger is actually getting then he can if necessary think about a larger capacity charger and just how fast he needs to recharge batteries. olewill

 

[geem]

I dont think this is a straight forward answer without more information. As a rule of thumb you can charge at 20% of the battery capacity without doing any harm. So of you had a 1000amp domestic bank then you could charge at 200 amps. If you are using wet cell batteres then you would typically be able to charge them at 14.8v without a problem. A good marine battery charger will allow you to pick the bulk charge voltage. If you have two chargers where you can set the same voltage then great. The batteries will not know the difference. If one charger has a slightly lower bulk charge setting then they will both run until this lower setting is achieved then that charger will start to back off its charge amps. If the combined charge output is greater than the 20% capacity of the batteries then there is a risk you will strain your batteries by boiling them excessively. Lots of marine chargers allow you to set the charge rate so if you are using two chargers you can set one of them down to give you the combined 20% of battery capacity output as a maximum. Once the batteries are getting charged you can turn off one of the chargers as the neccessary output will be available from a single charger. The Sterling Ultra chargers specifically say you can use more than one charger but I see know reason why you cant mix and match manufacturers as long as you obey the simple rules

 

[PaulRainbow]

The Sterling Ultra chargers specifically say you can use more than one charger but I see know reason why you cant mix and match manufacturers as long as you obey the simple rules

Where does it say this ?

 

[geem]

Where does it say this ?

In their literature
"Larger requirements?
After your final calculations are made, if you require a larger charger, then the Sterling range, because they are digital chargers can be added together. For larger requirements simply add units (2-3-4 etc) together to meet your requirements. Multiple smaller units can offer some advantages over 1 larger unit: They tend to be lower cost, have a built in redundancy, (in event of one failing) and enables you to switch one off in the event of low shore power facility. "

 

[PaulRainbow]

In their literature
"Larger requirements?
After your final calculations are made, if you require a larger charger, then the Sterling range, because they are digital chargers can be added together. For larger requirements simply add units (2-3-4 etc) together to meet your requirements. Multiple smaller units can offer some advantages over 1 larger unit: They tend to be lower cost, have a built in redundancy, (in event of one failing) and enables you to switch one off in the event of low shore power facility. "

Couldn't find that in my literature, but found it online, thanks.

The issue, as i see it, is that one charger will detect the output from the other charger and think that the batteries are charged, thus it will shut down. Sterling seem to think this won't happen with their chargers but it absolutely does happen with other combinations. I've not specifically tried connecting two Sterling Ultra chargers in parallel, but i can say that if connected in parallel with a Victron solar controller the Sterling charger will cause the solar controller to shut down.

 

[geem]

Couldn't find that in my literature, but found it online, thanks.

The issue, as i see it, is that one charger will detect the output from the other charger and think that the batteries are charged, thus it will shut down. Sterling seem to think this won't happen with their chargers but it absolutely does happen with other combinations. I've not specifically tried connecting two Sterling Ultra chargers in parallel, but i can say that if connected in parallel with a Victron solar controller the Sterling charger will cause the solar controller to shut down.

The only reason your solar controller shuts down is that the set bulk charge voltage is lower than the set bulk charge voltage on the mains charger. I can see this on my set up as well but change the bulk charge voltage of the solar controller to be higher than the mains charger bulk charge voltage and the solar controller doesn't shut down.
If its sunny and my panels are putting plenty of charge in, I sometimes set my solar charger above the battery charger set voltage just to stop the solar charger shutting down. This is easily done with the Victron solar charger I use via my ipad.

 

[rogerthebodger]

The only reason your solar controller shuts down is that the set bulk charge voltage is lower than the set bulk charge voltage on the mains charger. I can see this on my set up as well but change the bulk charge voltage of the solar controller to be higher than the mains charger bulk charge voltage and the solar controller doesn't shut down.
If its sunny and my panels are putting plenty of charge in, I sometimes set my solar charger above the battery charger set voltage just to stop the solar charger shutting down. This is easily done with the Victron solar charger I use via my ipad.

I completely agree with geem

Paul the mistake you are making with this statement "one charger will detect the output from the other charger and think that the batteries are charged "

The one charger is detecting the battery voltage not the charger output. This is because the charger has am output resistance such that the resultant voltage is dependant on the current flow from the charger to the battery. This current flow really as a result of the internal resistance of the charger and battery to gether with the voltage difference between the no load charger voltage and the no charge/discharge voltage of the battery.

The charges will detect this resultant battery voltage and as the battery voltage rises as the battery charger state increases the different chargers will turn off or change charging state at each one reach its turn off voltage.

 

[PaulRainbow]

The only reason your solar controller shuts down is that the set bulk charge voltage is lower than the set bulk charge voltage on the mains charger. I can see this on my set up as well but change the bulk charge voltage of the solar controller to be higher than the mains charger bulk charge voltage and the solar controller doesn't shut down.
If its sunny and my panels are putting plenty of charge in, I sometimes set my solar charger above the battery charger set voltage just to stop the solar charger shutting down. This is easily done with the Victron solar charger I use via my ipad.

Mine are both set to the same voltage. It doesn't cause any issues in reality, because i don't have the mains charger switched on. I have battery monitoring and relays to turn the mains charger on automatically if the batteries fall below a set voltage.

 

[lw395]

None the less, there are plenty of chargers about which will see that the battery voltage is above a threshold, which might be below 13.8V and not start charging.
Making two power supplies work together is often a fraught business.
It would be wrong to just assume it will work.

On a small scale, I have a small Ctek and another trickle charger. If the trickle charger is connected first, the Ctek sees more than the voltage of a fully charged battery and waits for it to drop.
If you switch the Ctek on first, let it settle in current limit phase, then switch the other charger on, the other charger pitches in with some current.

But I think the little Ctek is designed with 'never overcharge' a log way up the requirements list.
Other chargers are designed with different priorities.