parallel chargers for more current?

[justDIY]

can I parallel common 12v battery chargers, to gain a faster charge time?

for example, three 160aH wet-cell batteries in parallel offers 480aH ... so 10% would be a little more than 40 aH, or two cheap 20amp chargers.

a single charger that can do 40a continuous is quite expensive compared to the 20a units available everywhere.

 

[Sceadwian]

No.

I'm sure there may be some out there, but I don't know of any lead acid charger topologies that allow for such a simple paralleling. The chargers basicaly would require some feedback between chargers in order to maintain current ballance, otherwise one or the other charge would become unballanced and start a possibly proprietary mode which would fry one or the other charger, or the battery.

 

[kchriste]

It'll work if they are just "stupid" chargers with no charge control cicuitry. (Basically a transformer and rectifier)
Using two "intelligent" chargers will work fine during the "bulk charging" mode. Once the battery voltage reaches around 14.2V then one of the chargers will cut back on the charge current before the other one will. Depending on the chargers used, anything from "just one charger supplying the current" to "them oscillating the charge current on and off between them" to "total meltdown" as depicted above could happen.
Try it out while they are still under warrenty.

 

[ljcox]

can I parallel common 12v battery chargers, to gain a faster charge time?

for example, three 160aH wet-cell batteries in parallel offers 480aH ... so 10% would be a little more than 40 aH, or two cheap 20amp chargers.

a single charger that can do 40a continuous is quite expensive compared to the 20a units available everywhere.

1. you can't charge batteries in parallel as it would be impossible to ensure that the same current is going into each battery.

2. the issue with connecting 2 chargers in parallel is that they will not both output exactly the same voltage. Thus one will supply more current than the other and one may even damage the other.

So in order to use 2 chargers in parallel, you would need a current balancing circuit. In order to do this, I suspect that you would have to modify the chargers so that the correct feedback can be applied to both chargers (independently) so that they both supply the same current to the battery.

 

[mvs sarma]

Hi Sceadwian

it is perfectly right. this technique is ude in telecom where one charger can ve on voltage mode and additional cahrgers will be working at slightly higher voltage but current limited mode. there is a sequencial switching arranged depending on the current drawn by the first charger(voltage mode) whether the current drawn by it is less then 20% of predefined load --switch off one slave charger-- if more than 80% then switch on additional charger--

you may have to design this intelligence into the set of chargers.

 

[ljcox]

Hi Sceadwian

it is perfectly right. this technique is ude in telecom where one charger can ve on voltage mode and additional cahrgers will be working at slightly higher voltage but current limited mode. there is a sequencial switching arranged depending on the current drawn by the first charger(voltage mode) whether the current drawn by it is less then 20% of predefined load --switch off one slave charger-- if more than 80% then switch on additional charger--

you may have to design this intelligence into the set of chargers.

HOWEVER, as I said in my previous post, you cannot charge batteries that are connected in parallel.

 

[justDIY]

HOWEVER, as I said in my previous post, you cannot charge batteries that are connected in parallel.

What evidence do you have to support these claims? I've been charging batteries in parallel, without any problems, however, only using a single charger.

As I understand it, as the lead acid battery nears a complete charge, its terminal voltage increases. This increase reduces the amount of current that battery will take, assuming of course, the charger is voltage regulated. As different batteries in the parallel bank reach full charge, batteries with less charge end up getting more current until they all equalize at whatever voltage limit the charger is set to. I can see where float charging with parallel chargers might be a problem. I can build up a comparator circuit that will disconnect the extra charger once the terminal voltage reaches a certain point (2.4v/cell for example) and let the single charger carry the bank to completion.

Of course, I can see there being issue if one of the batteries is damaged or overheated, and is unable to equalize its voltage with the other batteries - then it will be subjected to a large current as the charger is trying to raise its voltage. However, I'm not looking for a turn key automatic system here. I just want something to fast charge a battery bank for a few hours while I have access to ac power.

 

[HiTech]

from: **broken link removed**


**broken link removed**

Two Batteries in Parallel, One Charger

Batteries connected in series strings can also be recharged by a single charger having the same nominal charging voltage output as the nominal battery pack voltage. In Figure 8, a single 24-volt charger is connected to a 24-volt battery pack.

In Figure 9 we see a pair of 12-volt batteries connected in parallel. This 12-volt battery pack is connected to a single 12-volt charger. Note the blue wire designated W1. The purpose of this wire is to balance the voltage drop evenly across both batteries and each wire during charging. This is not critical for lower current chargers, but when you start to get into the 10 amp and above range, the voltage differential can be significant. The blue wire W1 must be connected to the opposite end of the battery pack as the black wire at the top of the battery pack.

When batteries are connected in parallel, only use one charger. Do not connect a charger to each battery, unless you break the electrical connection between the batteries. The reason is that the chargers will very likely complete one or more their charging subroutines (charge modes or stages) at different times. That means that each charger would be trying to bring the battery pack to a different voltage level. Depending on how the chargers are configured to prevent a reverse polarity connection, the charger with the lower voltage output could possibly draw current from the charger with the higher voltage output, or even from the battery pack that it is trying to charge. If the chargers' reverse polarity protection mechanism includes a solid state, unidirectional, voltage controlled, current switch (like a diode), then this is not a big problem.

 

[ljcox]

What evidence do you have to support these claims? I've been charging batteries in parallel, without any problems, however, only using a single charger.

As I understand it, as the lead acid battery nears a complete charge, its terminal voltage increases. This increase reduces the amount of current that battery will take, assuming of course, the charger is voltage regulated. As different batteries in the parallel bank reach full charge, batteries with less charge end up getting more current until they all equalize at whatever voltage limit the charger is set to. I can see where float charging with parallel chargers might be a problem. I can build up a comparator circuit that will disconnect the extra charger once the terminal voltage reaches a certain point (2.4v/cell for example) and let the single charger carry the bank to completion.

Of course, I can see there being issue if one of the batteries is damaged or overheated, and is unable to equalize its voltage with the other batteries - then it will be subjected to a large current as the charger is trying to raise its voltage. However, I'm not looking for a turn key automatic system here. I just want something to fast charge a battery bank for a few hours while I have access to ac power.

My point is that, with parallel batteries, the batteries are unlikely to be identical even if bought together from the same shop, thus you can't know whether the same current is going into each battery and therefore you don't know how long to keep charging. I agree that the current will decrease as the batteries charge, but it is the uncertainity that concerns me.

As for parallel chargers, you don't know what current is being delivered by each charger since the chargers won't be identical either.

 

[Hero999]

1. you can't charge batteries in parallel as it would be impossible to ensure that the same current is going into each battery.

Yes you can providing it's constant voltage charging not constant current. I've done it before, I set the power supply to 13.75V (the rated float charge voltage for the batteries) and connected them in parallel.

However you're right in that you can't parallel most battery chargers.

 

[ljcox]

Yes you can providing it's constant voltage charging not constant current. I've done it before, I set the power supply to 13.75V (the rated float charge voltage for the batteries) and connected them in parallel.

However you're right in that you can't parallel most battery chargers.

Yes, that would be alright for float charging, but as I understand it, he is not talking about float charging, he wants to charge the batteries and then disconnect the charger. My problem is - how would you know when both batteries are charged.

 

[Sceadwian]

mvs sarma, yes, that is what I said. That kind of charging scheme will not work unless there is some kind of transfer of charge state voltage and current information between the chargers (feedback)

Mind you if you just set the constant voltage level and connect a few chargers in parrallel it will work, it may also burn out one or another charger as individual batteries may be significantly out of ballance with one another. This will cause a voltage imballance between chargers and therefore a current transfer, only limited by the transformers.. It also makes a LOT of heat. You can insert diodes to combat this, but it eliminates feedback. You need basically the individual voltage of every cell (sub cell in a serial lead acid configuration) and some method of judgeing the current into/outof each cell as well. You need a smart battery, and a smart charger. S'why the first line on my responce post was "No" Because it simply does not work that way. Two 12 volt lead acid cells in series are actually 6 2.1 volt cells in parallel with 6 more 2.1 volt cells in series. So you're really dealing with a 6 series 2 parralell configuration. Batteries get more 'complicated' with more series/pararllel connections, chargers get 'simpler' more seriers/parallel configurations.

 

[Dialtone]

Well I hate to burst everyone's bubble. but, not only is it possible to charge multiple batteries, but also do it using multiple charging rectifiers.
I have done both for many years in telecom applications.
I will grant you that the rectifiers used were both voltage AND current limit controlled, so some cheap stand alone charger probably would not behave well if paralled with another charger, but with the right equipment, anything is possible.

Most of the parallel charging systems suggest you employ a (large) diode in the output to isolate the rectifiers from feeding back into each other, but it is not necessary unless you want to "hot swap" them.

If you want to see multiple rectifer applications, Google Kepco HSP48-30 and read the info on these and others in their product line. (no I do not work for them, so no product plug intended)

 

[mvs sarma]

HOWEVER, as I said in my previous post, you cannot charge batteries that are connected in parallel.

YES, Ijcox is right.
i was only telling about chargers being in parallel.

 

[ljcox]

can I parallel common 12v battery chargers, to gain a faster charge time?

for example, three 160aH wet-cell batteries in parallel offers 480aH ... so 10% would be a little more than 40 aH, or two cheap 20amp chargers.

a single charger that can do 40a continuous is quite expensive compared to the 20a units available everywhere.

Going back to his original question (as above), ie. does two cheap 20A chargers = a 40A charger, the answer is no.

The two 20A chargers won't be identical due to component spreads and therefore one will deliver more current than the other. Thus if a total of 40A is being delivered, one may be delivering say 25A and thus the other will be delivering 15A. Hence one is overloaded