n00B question about blocking diodes and charging multiple batteries from one charger

Thread Starter

electrohippy

Joined Jun 13, 2018
9
Sorry folks, I'm unclear as to how to search the subset of power supplies/charging for answers to my question, so not sure if this has already been answered! This is probably basic anyway...

I want to charge multiple small 12v batteries at the same time from one big 12v charger/maintainer without damaging the batteries. They will come to the charger at the same time each week but at different levels of charge, and will stay on the charger for the next week until swapped again, hopefully in maintain mode.

Basic question: Can I just add blocking diodes on different "outputs" to "split" the charger/maintainer to charge/maintain each of the batteries independently, and what would such a circuit look like? I have attached a .jpg of what I think it should be, but please correct me if I'm wrong! It's OK if this isn't a "perfect" solution, but "absolutely unworkable" is not OK...

Thanks in advance!!!
 

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Thread Starter

electrohippy

Joined Jun 13, 2018
9
Oh yeah, for clarity, I'm talking about 12v 12.5AH SLA batteries and an off-the-shelf automotive battery charger/maintainer, and the system needs to be "plug-and-play/set-and-forget because unskilled helpers will be handling the batteries most of the time.

Thanks!
 

AlbertHall

Joined Jun 4, 2014
8,321
When you connect a flat battery all the charge current will flow to that battery so the charger must be suitable for connection to just one battery (don't use a larger current charger).

There will be some voltage loss across the diodes but as long as you use schottky diodes this will be minimal and not cause any problems.
 

Thread Starter

electrohippy

Joined Jun 13, 2018
9
Ah... So let me see if I get you... what you're saying is that the batteries will actually be charged in succession starting with the "flattest," and not simultaneously, even if I connect all four at the same time... So I should use a charger/charge rate (amps) that's appropriate for a single 12v 12.5Ah battery and that will work fine for four batteries connected at once since it's only really charging one battery at a time, assuming I have the proper schottky diodes in place per my diagram?
 
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AlbertHall

Joined Jun 4, 2014
8,321
Ah... So what you're saying is that they will actually be charged in succession, not simultaneously... So I should use a charger/charge rate (amps) that's appropriate for a single 12v 12.5Ah battery and that will work fine for four batteries "at once," assuming I have the proper schottky diodes in place per my diagram?
Yes. The lower voltage battery will charge until its voltage is equal to next lower battery voltage and then the two batteries will share the charge current and so on until all batteries have the same terminal voltage when all will share the current.
[EDIT] This system was used on a narrow boat I was acquainted with. The generator on the engine charged two batteries via diodes. One battery was used for starting the engine and the other for lights, fridge etc. If everything was run from one battery then you could run down that battery to the point where it wouldn't start the engine and so you couldn't recharge the battery.
 

Thread Starter

electrohippy

Joined Jun 13, 2018
9
Thanks for the info!!!

Will the batteries be damaged or have their life expectancy lowered if charging is done this way as opposed to actual individual chargers on individual batteries?
 

Thread Starter

electrohippy

Joined Jun 13, 2018
9
I ask because I (sort of) understand that using batteries of different ages in DC systems such as solar or RV/boat systems is a no-no because, as I understand it, the old batteries will effectively damage the new ones. Since all the batteries in the system I am working out here will be discharged at different rates, more or less making each one a different "age" (as I see it), it seems that every time they are connected to the charger (in groups of four at the same time each week), there is the potential that this type of inter-battery damage might occur. I'm trying to avoid said damage between batteries. So will the diodes afford that protection (assuming I'm correct about odd-aged batteries and damage)?
 
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AlbertHall

Joined Jun 4, 2014
8,321
Connecting odd-aged batteries in direct parallel (or indeed in series) is a bad idea.
this problem is avoided by the diodes.
 

crutschow

Joined Mar 14, 2008
23,310
Adding the diodes will likely cause the batteries to not be fully charged due to the forward voltage drop of the diodes.
 

AlbertHall

Joined Jun 4, 2014
8,321
Adding the diodes will likely cause the batteries to not be fully charged due to the forward voltage drop of the diodes.
12V SLA with schottky diodes?
When fully charged the current will be low?
I don't think they will make a significant difference.
 

AlbertHall

Joined Jun 4, 2014
8,321
The variation in voltage due to the diodes is less than the possible variation due to temperature and it would be pretty rare for the battery temperature to be measured by the SLA charger.
 

ebp

Joined Feb 8, 2018
2,332
The single most critical thing with SLA batteries is to be sure the charging voltage is not excessively high, especially for "float" charging where the batteries are left connected to the charger well after they have reached full charge. The general range is 2.25 to 2.35 V per cell. Ideally, temperature compensation is also used, but it isn't necessary if the charging is done at "room temperature."

Current limiting is generally a good idea, but the reality is that a battery that is discharged but in good condition that is connected to a constant voltage charger at float voltage is not going to take excessive current. To really slam high current into lead acid batteries it is usually necessary to raise the charging voltage considerably above float voltage. It is worth doing some testing to confirm that the batteries won't accept excessive charge current with a float charger that can deliver enough current to charge a large or multiple batteries. This can be a more or less insoluble problem when charging multiple small batteries with various states of charge of a single large charger, and diodes will do little to improve matters, other than by dropping the charging voltage a little.

Connecting a charged battery and a deeply discharged battery directly together is probably OK, but again should be verified by monitoring current. In a test setup I would use a power resistor of an ohm or so between the batteries when they were first connect to limit the current. If it looked tolerable, then I would either proceed to a lower value resistor or a cautiously made direct (through an ammeter) connection. Diodes certainly prevent this problem.

Mixing old and new batteries in a series connection is definitely a bad thing to do. Paralleling them, provided the old battery doesn't have a shorted cell, generally won't harm anything, again with the caveat of possible excessive current for awhile. Really "worn out" batteries tend to rise to float voltage very quickly. Putting any battery with a shorted cell on a charger that can't detect it is inviting disaster. With SLAs it will very likely blow the seals on the good cells. That may or may not blow electrolyte over the landscape.

Although it is inefficient, one possible solution would be to use a low value resistor between the charger and each battery. This would improve current limiting to the individual batteries and probably result in less reduction of float voltage than a diode since the charge current would become quite low as fully charge was reached. It would of course take longer to charge the batteries. If I were doing it, I'd probably use either tubular ceramic wirewound resistors or the aluminum-housed types that can be mounted to a heatsink, and mount them all on an aluminum plate. I might add a fan.

Don't forget that the diodes may get pretty hot.
 

crutschow

Joined Mar 14, 2008
23,310
The variation in voltage due to the diodes is less than the possible variation due to temperature ....
A 0.5V change in battery voltage corresponds to over a 100°C change in temperature for a 12V lead-acid battery, so I don't know how you came up with that statement. :confused:
 

ebp

Joined Feb 8, 2018
2,332
I generally used a tempco of -5 mV per degree Celsius per cell when I was designing chargers, but I've seen considerable variation in recommendations.
 

Thread Starter

electrohippy

Joined Jun 13, 2018
9
Thanks everyone who chimed in!

So then, it looks like I may just be overthinking this. I had assumed that if batteries were just directly connected in parallel to the charger, there would be inter-charging between batteries if some were low charge and the others high charge, and that this was a bad thing and would damage them in short order. That was my reasoning for the idea to place schottky diodes between batteries and charger. Or is it that this is true, but resistors would work better?

So how bad would it be for battery lifetime if, every week, I attach all four batteries in parallel directly to (no added diodes or resistors) something like this "NOCO Genius G750 6V/12V .75A UltraSafe Smart Battery Charger," then swap them to the load for the second battery set after one week, put the second set on the charger... rinse, repeat? Will I end up with batteries that won't hold a charge in a few months?
 
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Thread Starter

electrohippy

Joined Jun 13, 2018
9
Sorry, I copied the wrong title for the charger, it's actually this one:

NOCO Genius G3500 6V/12V 3.5A UltraSafe Smart Battery Charger

which says it's "A 6-volt and 12-volt battery charger and maintainer for lead-acid and lithium-ion batteries up to 120 amp-hours"

The "NOCO Genius G750 6V/12V .75A UltraSafe Smart Battery Charger" says of itself "A 6-volt and 12-volt battery charger and maintainer for lead-acid batteries up to 30 amp-hours."

Since the combined total of all four batteries is 50Ah (12.5Ah x 4 in parallel = 50Ah), the G750 would be insufficient to charge them together according to their specs... right?

Thanks again for all the help everyone!
 

ebp

Joined Feb 8, 2018
2,332
Diodes will completely prevent one battery from discharging into another, so that is probably the better way to go. They would slightly reduce the charge voltage. You might compromise by putting resistors across the diodes. 10 ohm resistors would probably be about right. They would allow some "cross charging" but also be low enough that by the time the batteries were approaching full charge the voltage across them would be quite small. The diodes would carry most of the charging current until the batteries got nearly topped up. I'd probably experiment a bit with resistors in about the 5 to 30 ohm range. The power rating of the resistors would depend on the value ultimately chosen and the maximum difference in voltage between batteries. If 10 ohms were used and the max difference were 6 V, the power rating would need to be 6^2 / 10 = 3.6 W. 5 W wirewound resistors are fairly cheap, but the will get quite hot at 3.6 W. [EDIT] Discharging an SLA below 10 V is being pretty mean to it, so the maximum voltage you would likely see across a resistor would almost certainly be less than 3 V. This would favor going with lower resistance which would mean even less loss of voltage for full charge.

Any of those chargers up to the 4.4 A model would probably be safe in terms of putting all of its output current into a single 12 Ah battery, but it would be wise to check the battery specs for maximum allowable current. The 750 mA model would certainly be safe in that regard.

The "diagnostics" and "recovery" features might actually complicate things, but they may prevent problems (other than not getting your good batteries charged) if one of your batteries had a shorted cell.

The 750 mA version would probably take about 3 days to fully charge 50 Ah of batteries that had been "fully" discharged. Unless there is some timer function built into the charger, that shouldn't be a problem if you intend to leave the batteries on the charger for a week.
 
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crutschow

Joined Mar 14, 2008
23,310
I generally used a tempco of -5 mV per degree Celsius per cell when I was designing chargers, but I've seen considerable variation in recommendations.
Okay, I see I miscalculated by using the one cell value for the complete battery.
Battery University gives a value of -3mV/°C.
Using that, 0.5V would correspond to a 27.8°C (50°F) change in a 12V (6-cell) battery.
 

Thread Starter

electrohippy

Joined Jun 13, 2018
9
Thank you thank you thank you to you all!

I think I'm set, I really appreciate the help.

CHEERS!
 
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