Parallel charging of different capacity batteries

[Paul_NJ]

I’d like comments upon parallel charging batteries of same chemistry, but different capacities. I’ve read statements pro and con. I’ve got 2- 116 amp-hr primary batteries hard wired in parallel being charged by a MPPT controller. Those batteries are used to power 12vDC lighting. I’ve also been connecting multiple (6 to 8) rechargeable lanterns, plugged-in in parallel with the primary batteries. These lanterns are rented out to people in remote villages, who return them for recharging. The lantern batteries are 7.2 amp-hr. All batteries are sealed lead acid AGM batteries, and the system operates at 12 vDC. The problem I’m having is that the lantern batteries don'’t seem to be getting fully charged and don't know if it is a problem in the lantern internal circuitry, or the parallel connection of all the batteries. Would it be better to build a separate system with panels and controller strictly for the lantern batteries? Why? Thanks

[inetdog]

Yes, it would be better to have a separate charge controller (CC) just for the lantern batteries, but it would have to have its own set of panels. It rarely works to have two CCs fed from the same parallel st of panels.
The basic ;problem with your charging scheme is that the lantern batteries cannot reach full charge voltage until your main batteries are also fully charged, and this is apparently not happening with the charging and load configuration your are running.
If you want to have only one set of panels, charging your main battery bank, your most reliable method would be to use a DC to DC converter running off your main batteries to supply a controlled higher voltage to the lantern batteries through a charge controller.
This will not be simple and is not likely to work with off the shelf components.

Another alternative would be to disconnect the CC going to the main batteries while you are using a second CC to charge the lantern batteries. You would only connect one CC at a time to the panels.

[jflorey2]

IThe problem I’m having is that the lantern batteries don'’t seem to be getting fully charged and don't know if it is a problem in the lantern internal circuitry, or the parallel connection of all the batteries.

As Inet mentioned you're not getting enough voltage to charge the batteries. When batteries are connected in parallel, the only way you can ensure that they are charged is through voltage - you have to keep the system at the appropriate absorb or float voltages to fully charge the batteries. Since your main battery and smaller batteries will never "agree" on when to switch from absorb to float you will basically have to keep the system at an appropriate float voltage (say 13.8 volts) for long enough to charge the small batteries, which is often ~24 hours.

[Sunking]

Since your main battery and smaller batteries will never "agree" on when to switch from absorb to float you will basically have to keep the system at an appropriate float voltage (say 13.8 volts) for long enough to charge the small batteries, which is often ~24 hours.

Maybe, maybe not. Only if the charger can produce a minimum C/10 current to the batteries. If less than C/10 will be longer and could be days to never happens.

However I strongly disagree you cannot go through 3 stages with mixed capacity batteries, because you can. This assumes the same chemistry type of battery Example say a Trojan t-105 in parallel with Trojan L16. Voltage is meaningless when charging batteries. Only thing voltage means is the Set Point at which stages occur. What is important is how much current the charge can produce, again C/10 is the magic number. As long as the charge current is no greater than C/8, and no lower than C/12, you can use any charge algorithm you want and have at your disposal. Only real difference is the amount of time and stress you put on the batteries.


If you have a C/10 charger, and the batteries are fully discharged it will take roughly 16 hours to recharge using a 3-Stage Algorithm, or roughly 24 hours using a single stage Float Algorithm. 3-Stage charges stress the batteries, and Float is kind and gentle.

EDIT NOTE:

Although you can charge mixed capacity batteries, does not mean you can cycle mixed capacity batteries. Battery warehouses all put their batteries on a Float Charger and all in parallel. It is common practice for long term storage. Once at Float voltage whatever that maybe, you can store batteries for quite a long time. Just remember batteries have two life cycles.

1 They have cycle life which you will never experience. Example Trojan RE batteries have a claimed cycle life of 3000 cycles to 50% DOD. That would make you think 10 years which is a fantasy.

2. All batteries have a Calendar Life which is the doom of 90% of battery failures. How long is that Calendar? Simple the warranty period. 1 to 7 years depending on the manufacture and quality of the product. Price per Watt Hour of capacity will also tell you. A 1 year battery will cost you roughly $80 to $100 per Kwh. A good 3 year battery like a Trojan T-105 will cost you $120 to $150 per Kwh. A good 7 year Trojan Industrial or Rolls series 5000 will cost you $220 to $240 Kwh.

[jflorey2]

Maybe, maybe not. Only if the charger can produce a minimum C/10 current to the batteries. If less than C/10 will be longer and could be days to never happens.

Agreed. You must maintain float voltage the whole time.

However I strongly disagree you cannot go through 3 stages with mixed capacity batteries, because you can. This assumes the same chemistry type of battery Example say a Trojan t-105 in parallel with Trojan L16. Voltage is meaningless when charging batteries.

Current is meaningless when batteries are paralleled; there is no way to know which battery is taking how much current, especially if they are different sizes/ages. That's why you cannot reliably switch from absorb to float, for example, based on current. You have to do it by time - which means that with a mixed battery bank you'll pretty much always get it wrong.

Voltage, however, is far from meaningless. Float a battery at 13.8 volts for 24 hours and you will be close to a full charge. This will happen due to the current flowing into it, and the only thing that will charge a battery is current. However, having the voltage set correctly causes that to happen. "Float" it at 10 volts (or 18 volts) and you will destroy it.

[Sunking]

Voltage, however, is far from meaningless. Float a battery at 13.8 volts for 24 hours and you will be close to a full charge. .

Complete nonsense Jeffery, I am surprised with you. Try connecting a 500 AH battery to a 10 Amp charger. You will likely die of old age before that battery is charged.

As for parallel with 3-stage current is all that matters as with any battery charger. Voltage is just a set point.

What happens to batteries when you parallel them up? Do not charge or discharge them, just parallel them up. . What happens?

Answer they equalize to the same SOC voltage period. Now apply a charge or discharge. What happens to voltage? They are still equal voltage at any point. Current will split each string, but batteries self regulate current. Batteries in parallel CANNOT HAVE different voltages, no parallel circuit can have different voltages. Current can be different, but that is a function of resistance. On paper if we parallel a 100 AH battery with a 10 AH battery, and draw 10 amps, what is the current in each battery? 9 and 1 amp. What is the voltage? Exact same SOC voltage.

If we Bulk Charge parallel or single strings at say C/10 on a 12 volt 100 AH total capacity . What is the voltage of the charger and battery? You cannot answer that question. Impossible to answer. Only correct answer is both batteries have the exact same voltage, but you cannot say what that voltage is. Both are at the exact same SOC voltage

If we set Bulk to 14.4 volts, once the batteries reach 14.4 volts, current is still C/10 for a moment. Current will taper off to 0 amps when the charger voltage and battery voltage are equal. Absorb is not a timed event no matter what you have read. Absorb stage terminates when current taper to C/33. At that point we switch to float and current stops and will actually reverse to bleed off any surface charge and wil settle at Float Voltage to what ever we have set it to, say 13.6 volts. The charger holds 13.6 volts and the battery Floats indefinitely. Both batteries will be at the exact same SOC Voltag during the whole charge process. It is simple battery Physics and Ohm's Law.

Voltage is only a set point, and yes it must be set correctly, but charger set point voltage only equals battery voltage when no current is flowing. Battery voltage is always lower than charger voltage except when current flow stops.

[jflorey2]

Complete nonsense Jeffery, I am surprised with you. Try connecting a 500 AH battery to a 10 Amp charger. You will likely die of old age before that battery is charged.

That is because that 10 amp charger may not be able to maintain 13.8 volts. If it can, it doesn't matter what current the charger is supplying. The voltage - 13.8 volts - will float charge the battery. If the charger cannot maintain the voltage, then the voltage will be incorrect, and thus the battery will not charge properly (which is what he is seeing.)

It is the VOLTAGE that matters in this case (parallel connected dissimilar batteries.) 13.8 volts will work. 10 volts won't. 18 volts won't. The current is important only in that the charger must be able to provide enough current to maintain that voltage.

Don't believe me? Try it yourself. Put a discharged 12 volt lead-acid battery on a good 13.8 volt voltage source, say a lab supply. Wait 24 hours. See if it charges.

Now do the same thing with a 10 amp (or a 100 amp) constant current supply. Put the battery on the charger. Wait 24 hours. See what happens.

[inetdog]

I think that Dereck (Sunking) will agree that unless you have enough charging capacity to reliably bring your main batteries (which are being cycled every night) up to full 100% SOC well before the end of the day. you will not be able to get the lantern batteries up to full charge by putting them in parallel.

It is nearly impossible to bring a cycled battery bank up to full charge within the sun hours available in any given day unless you have a lot more panel wattage than you need just to replace your energy consumption. That makes it hard to bring your lantern batteries up to full charge even if the amount of energy that they take is relatively small.

[Sunking]

That is because that 10 amp charger may not be able to maintain 13.8 volts. If it can, it doesn't matter what current the charger is supplying. The voltage - 13.8 volts - will float charge the battery. If the charger cannot maintain the voltage, then the voltage will be incorrect, and thus the battery will not charge properly (which is what he is seeing.)

It is the VOLTAGE that matters in this case (parallel connected dissimilar batteries.) 13.8 volts will work. 10 volts won't. 18 volts won't. The current is important only in that the charger must be able to provide enough current to maintain that voltage.

Don't believe me? Try it yourself. Put a discharged 12 volt lead-acid battery on a good 13.8 volt voltage source, say a lab supply. Wait 24 hours. See if it charges.

Now do the same thing with a 10 amp (or a 100 amp) constant current supply. Put the battery on the charger. Wait 24 hours. See what happens.

Done it professionally for over 35 years. I know exactly what happens. I even sat on IEEE 484-2002 and 485-2010. So you can say I helped write the book on batteries. A privilege I earned being the largest battery customer in the world for 6 years in the telecom sector. You must not understand how a battery charger works.

No charger can maintain a 13.8 volts if the current demands exceeds the charger current limit. Simple Ohm's Law. Let's try this with some real working values. Let's say I have 12 volt 225 AH battery at 30% SOC Open Circuit Voltage = 11.75 volts. The battery Ri = .005 Ohms. I take a 2 amp charger or bench power supply and set the voltage to 13.8 volts. Connect it to the battery.

Assuming the wires between charger and battery term post is 0 Ohms. what is the Charger and Battery voltage? You had better not say 13.8 volts. You had better say 11.76 volts.

How many hours will it take for the battery voltage to reach 13.8 volts? You had better say a minimum of 79 hours or just over 3 days. Turn that voltage up to infinity and it will still take 79 hours to reach 13.8 volts.

If you came up with any other answers, then you do not understand batteries.

Here is a hint. Lets say I have a 13.8 volt supply of infinite current, same battery, same SOC of 11.75 volts, same Ri .005 Ohms. I connect the charger to the battery, now both battery and charger are at 13.8 volts. What is the charge current? You had better say 410 amps.

How long will it take to charge the battery with 410 amps? OK that is a trick question because the battery will explode.

[Sunking]

I think that Dereck (Sunking) will agree that unless you have enough charging capacity to reliably bring your main batteries (which are being cycled every night) up to full 100% SOC well before the end of the day. you will not be able to get the lantern batteries up to full charge by putting them in parallel.

Yep, solar is not capable of charging batteries to 100% unless sufficiently over sized. For most crank the CC voltage to as high as you can to force the charger to stay in Bulk aka Constant Current. You use your hydrometer to set the voltage, not the battery spec. At the end of the day, if you rSG i slow,. crank the voltage up. Most likely you will never be able to crank the voltage up high enough to read a 100% SOC SG reading. If by some chance your SG is too high at the end of the day, back off the voltage a little and try again the next day until you find the right voltage

Rolls, Trojan, US Battery have adjusted voltage set point for solar users because of chronic under charging. Battery charging specs were written from the POV you are using an AC charger of C/8 to C/12 charge current. Where you set Bulk/Absorb to 2.37 vpc and switch to Float 2.2 when current tapers to C/33. Absorb is not a timed event, it is a CC/CV event and terminates when current tapers to C/33 at the specified voltage. Absorb takes 4 to 8 hours. Solar cannot possible do that. Manufactures got clobbered with warranty claims from RE users for following manufacture directions. So today they now call it Daily Charge of 2.45 vpc. CC/CV mode. That forces your Controller to stay in Bulk or CC mode all day unless you just happen to be over sized for the Absorb time to time out and cut back to Float of 2.2 vpc.

The takeaway here manufactures want you to run the battery on the Corrosion side of the knife edge (over charged). They do this because your battery will last longer than when the battery is on the Sulfate side of the Knife edge (under charged) You have to be on one side or the other. You cannot dance on the knife edge in a CYCLE OPERATION. Only folks who can dance on the knife edge is Emergency Stand By users on FLOAT CHARGE.

One last hint. When you connect a Voltage Source like a Power Supply or Battery Charger with limited current to a battery, you change your Voltage Source to a Current Source.

[Paul_NJ]

I’ve found this discussion very informative. I am new at this. I will try to repeat back what I think I have interpreted from it: please correct me if I’ve got it wrong.

Here’s my present system: Solar panels: combined 620 watts, capable of 620w/12v = 50 amps. Controller: Morningstar 45 amp MPPT 3 stage controller, so it can (almost) handle the full current. Batteries: (2) 116 amp-hr batteries and (8) 7.2 amp-hr batteries, all connected in parallel. They all are same chemistry (LA, sealed AGM). I attached copies of the battery spec sheets.



The controller will begin in bulk mode, and pump constant power into the parallel connected battery array until the voltage reaches the controller setpoint of 14.3 volts. Because the batteries are connected in parallel, they by definition will be at the same voltage. That voltage will be some combination of the batteries open circuit voltage plus the voltage drop due to current flow. Because the batteries don’t necessarily have the same SOC, the current flowing to each will not be the same.

As current flows into the bank, current will be distributed among the batteries until they all reach the same SOC. Current will drop as the batteries charge (SOC increases), and as they charge the open circuit voltage of the batteries increases (and the voltage drop decreases). My understanding is that this controller will stay in bulk mode however long it takes until a 14.3v setpoint is reached before passing into absorb mode, where the batteries will remain for an additional 2 to 2.5 hours at 14.3 v. When the battery is fully charged, the current will have dropped to some low value, and then the controller places them into float mode.

During all of this there’s no way to know what the current is to each battery. All we do know is what happens to the battery array as a whole. Total power is constant at 620 watts. If the battery array voltage starts out at 12v, the total current will be 620 watts / 12v = 52 amps. But that is distributed among all of the batteries. The only concern is that the batteries are not charged so fast that overheating can damage them. A common limit is that the current to each battery does not exceed C/8 amps.

This is the manufacturer's charging spec sheet for my batteries.



It states that bulk mode should continue to the battery cell voltage reaches 2.40 to 2.43 volts (14.4-14.5 volts total). Hence the controller’s 14.3 volt setting. The spec sheet also says the max charge current should be C/3 amps. It also gives an equation for the charge time as 1.2 x DOD / current.

The total of the 10 batteries is (116 x 2) + (7.2 x 8) = 290 amp-hrs. Going by their guideline, the max charge current that should be applied to the batteries would be 290 amp-hrs / 3 = 97 amps. So if the 50 amps of current the panels are supplying divides among the batteries according to their capacities, that limit won’t be exceeded. In reality, with different battery SOC’s, there’s no way to know if that will actually happen. According to the spec sheet, the charge time for the entire array, assuming all batteries are equally completely discharged would be 1.2 x 290 amp-hr / 50 amp = 7 hours. In Malawi, there are only 5 equivalent sunlight hours per day, so that is not going to happen.

So, from all of this, what I gather is that our solar array doesn’t generate sufficient power to charge all of the batteries in a day. So the batteries are not receiving full charge. What I should do is set up a separate system for the lantern batteries alone, with dedicated panels and controller, of sufficient power generation for them alone.

Am I getting this correctly??

Thanks for your continued guidance!

[Sunking]

Parallel charging batteries is fine for Stand By or Storage. I made that quite clear from the beginning. But for daily cycling applications is not recommended. You are going to experience short battery life with what you have.

[Mike90250]

the perils of parallel batteries.
> Batteries: (2) 116 amp-hr batteries and (8) 7.2 amp-hr batteries, all connected in parallel.
and mixing sizes this varied, you are asking (loudly) for trouble, when these burn (and I mean burn like fire) out, simply replace with a pair of 6V golf cart batteries (you can get sealed AGM cart batteries) and wire them in series. then no troubles.
When summer and longer days come, those 7.2 ah cells will get full and start to gas and vent, effectively destroying them,