Considering you will be hooking up the Batteries in parallel they should be the same. Two unequal batteries will discharge at different rates and recharging will most often result in cooked cells.
If you search the internet there are volumes written on this subject.
Edit: When I said the "same" I was referring to the construction and capacity of each battery. Also it is best that they both be of the same age. The Brand on the top has little to do with performance. Mixing a fresh battery with an aged battery can just cause some unhappy cells when recharging, resulting in some being undercharged and others being overcharged.
FYI, What you are doing is a duplication of the dual battery setup in most Diesel Pick-up's
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You can mix manufacturers if needed, but you should not mix different types ( deep cycle & Starting) , chemistry wet cell /AGM or capacity (CCA).
CCA and MCA are two standards for measuring battery capacity CCA is cold cranking amps. It is also recommended to keep the age of the batteries as close as possible. The issue with using different batteries is that they do not charge and discharge at the same rate therefore one may not be charged properly and could fail prematurely.
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FWI most people say replace both with equal at the same time. This is in an alternator auto/truck/equipment world. Not sure about RVs. I know if you have a bad battery in this case it will starve the good battery and try to force charge the bad battery. Can't explain it, but it does.
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I want to add another battery to my camper. I currently have an Interstate SRM24 550 CCA 690 MCA battery. Snapshot of my Interstate attached.
I saw some Marine Deep Cycle Group 24 batteries on sale at SAMS CLUB today for a heck of a price.
Do I need to buy another Interstate battery or can I mix these. I don't have a clue about what CCA and MCA means. If I did get another brand, should it also say 550 CCA and 690 MCA?
CCA is cold cranking amps (0 degrees) and only useful for starting boat engines when cold out.
MCA is Marine Cranking Amps (32 degrees) and only useful for starting boat engines when slightly warmer.
The real number that is important on that cheap battery that pertains to your RV is the RC (Reserve Capacity). RC is the number of MINUTES that your fully charged battery will last at 80 degrees when discharged at 25 amps.
To convert RC to AmpHours (the measure that is important in storage batteries used in RVs) you need to convert by multiplying RC by 0.4167.
AH rating is based on the delivery of amps for 20 hours (the 20 hour rating); so a 100AH battery will deliver 5 amps for 20 hours. Discounting something called the Peukert Effect; at 20 amps your 100AH battery will last 5 hours; etc ...
Thus your Group 24 battery has 140 minutes x 0.4167 or 58.3 Amp Hours.
Most cheap OEM Group 24 batteries (like my DC-24 Dekas) have a 75AH rating.
My Interstate battery is about 4 months old. Barely used, maybe 10 days use at most. I brought it home and stored it in the garage while we are in storage for winter. So I'd consider that new.
The new battery I'm considering is way cheap, marked down at least $40 to $35? Something like that. I don't see how I can lose at that price.
Actually, you can lose. I did. I had 2 matched Interstate Group 24 12V batteries in parallel, as delivered from the dealer.
To make a long story short, batteries were separated and recharged separately, apparently to very different charge states. I believe one battery may have had a bad cell. When I reconnected the batteries in parallel, I started an electrical fire in the battery box. The paralleling cables glowed cherry red, and the off-gassing hydrogen from the low battery flamed at the cables and at the vents (very pretty blue flames).
I was able to grab a pair of lineman pliers (insulated handles) and cut the paralleling cables. This put the fire out, but not before holes were made in the top of the battery cases, and the cables (6 gauge wire) were charred and melted. There was no detectable acid spill.
The batteries were used as core turn-ins for a set of Interstate 6V 232AH golf cart batteries at Costco. Total price for the 2 new batteries was $150, including sales tax. The cabling was replaced with a series set of cables, the camper cables cut back and new lugs installed, and a battery disconnect added on the outside of the battery box.
I am a much happier and safer person with the dual 6V setup, and I have more capacity to boot. I added a voltmeter inside to monitor converter or battery status (which it monitors depends on whether we are plugged in or not). After the change out from WFCO to a Progressive 4135 converter/panel, it truly is a reliable, capable system that I just don't worry about.
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Thanks Herk7769! Great explanation. It seems what I need to get are batteries with a similar RC value to get equivalent Amp Hour based batteries.
Actually what you need is batteries that give you a 20 hour rating AND are not dual purpose batteries that tout their starting capability AND their deep cycle capability.
ANY battery that does not call out a 20 hour capabilty may be considered a starting battery or a dual purpose battery.
RC capability is NOT a substitute for deep cycle performance. It is simply a measurement of how many amp hours you can expect. THE REAL problem is life cycles. HOW MANY TIMES can you discharge to 50% before the battery fails? This is largely a function of the thickness of the battery plates. Thin is good for starting...thick is good for LASTING. Any battery without a 20hr. rating from the mfr. is not suitable as a deep cycle house battery. Deep cycle batteries are generally a bit more expensive in the same size range compared to a starting or dual purpose.
You should expect 20hr. ratings for wet cells of: 75 amp hours from a group24, 95 from a group 27 and 105 from a group 31 battery that is deep cycle. Slightly more or less is possible... but beware of claims more than 10% different as the copy has been through the marketing department rather than engineering!
Never mix different types & sizes. They will work...but not as long or as well. Almost new with brand new is probably not going to affect anything.
Most R/V's come with dual purpose batteries which are fine if you are always plugged in somewhere like me. If you boondock...run those suckers into the ground and then replace with true deep cycles...6V or 12V.
I agree with the posts on this subject.
Yes it is OK to use different brand batteries in parallel.
Yes having batteries of same age and capacity ratings is a consideration.
But I would like to explain how the electricity works.
Having 2 6 cell 12vdc batteries in parallel is like having 6 separate batteries in series connected that are in parallel with another 6 separate batteries in series.
Within a 12vdc battery the 6 2vdc cells add up to 12vdc.
When all the cells are fully charged you get the maximum voltage from your 12vdc battery. If 5 cells are at 2vdc and 1 cell is at 1.5vdc, then the total battery voltage will be additive... 11.5vdc. This is an example, but typically the cell voltages usually are close to th same unless you have a bad cell.
When you load a battery you consume charge (current...) and the voltage drops. Within a 6 cell battery the same current flows thru all 6 cells, but the weakest cell is the bottleneck.
When you charge a battery the charge rate is based upon the voltage level and the charger current capacity.
If your 2 batteries in parallel voltage is 11.3vdc and your charger is a 10A charger it will try to increase the battery voltage to about 13.7vdc to 14.4vdc. But it will limit the current to 10A which in turn limits the voltage charge level.
When the battery is charging the voltage will slowly climb to the max charger voltage level but then the charging current will be minimal.
This is where the battery capacity and health come in.
If all the batteries in parallel are of the same capacity, voltage level and health, then the charging occurs evenly.
If one battery stand alone has lower voltage, then it will take most of the current from the charger AND FROM THE OTHER BATTERY until it is charged to where it can supply the same voltage as the other battery in parallel.
Voltage determines where the charging current goes. The lowest voltage battery hogs the current. Two 12vdc batteries in parallel will ALWAYS HAVE THE SAME VOLTAGE. But the current paths to them will be uneven if the batteries are not at the same voltage levels.
Think of it as 12 test tubes with water connected with two sets of six test tubes in series connected in parallel. The water level is the voltage and the water flow is current. The lowest water level (voltage) will demand the most water flow until it is level with the other battery. Since the cells within a battery are in series, they will all see the same same water flow (current), but old or unhealthy cells will require more flow to be able to maintain the voltage level. Think of a weak cell as having a small leak so it requires more flow to keep it level up. (This is just an analogy... No water/acid leaks!... Actually electron count is limited based upon cell plate health.) If the cell can't keep the water level (voltage) at a higher level then it would be reduce the total battery voltage for that battery. This or course means it will make this weak battery hog most of the charging current from the charger and it will lower the voltage of the other healthy battery in parallel with it by actually taking current from the healthy battery for the weak battery. In most cases, this just means charging will take longer and overall full charge capacity of the 2 batteries in parallel will be reduced (won't last as long...). In the extreme, the healthy battery could a actually be damaged as it is always having to provide current to the weak battery.
BUT... battery cells usually fail by their plates getting smaller or oxidized. This limits the current that cell can have flow through it and it becomes the charging bottleneck. It can still reach the "balanced" voltage level of the combined batteries, but it will limit it and reduce the time the 2 batteries can maintain voltage... (battery charge will not last as long).
So what did i just say?
You are most likely not going to hurt anything or cause safety issue with different brand or even age/capacity batteries connected in parallel.
But the more unbalanced, the lower your overall battery capacity and sustainable voltage levels will be. It will take longer to charge and you will not charge to the highest level.
AND... buying the cheapest battery is not the best battery. You still get what you pay for. HERK7769 stated this better than i could. I recommend buying the better battery and you will end up with longer battery life, less chrging time and overall it will cost you less because you will just have to replace the cheap batteries more often. But if budget it tight.. (it is with me..), then buy it and you are safe, but you will have to charge more and replace more often.
Don't mix battery types! wet cell / AGM / Gel pack
The post of the fire reported by pgandw is an extreme case where a fully charged healthy battery was put in parallel with a dead battery. When this happens there is no limit to the current that the good battery could provide to the dead battery except for the wires connecting the two batteries together which is why they burned. In this case literally hundreds of amps of current tried to flow from the good battery to the dead battery through these wires instantaneously. It was like welding...
The problem was probably not a bad cell in the dead battery. If that was the case, then it would have limited the current that the dead battery could draw and there would not have been a fire. He is lucky that the batteries did not explode. What I guess happened is that the dead battery was just that. A dead battery with no charge. There could be other reasons why it did not charge. Most bad batteries that will not charge are not able to pass this high of a current. It sounds like it was a good battery with close to zero charge.. (no voltage level)...
ALWAYS TAKE A VOLTMETER AND MEASURE EACH BATTERY VOLTAGE BEFORE YOU CONNECT THEM IN PARALLEL. If they are not within 1vdc to 2vdc of each other, then charge them separately until they are before connecting them in parallel.
pgandw: did you get a huge spark when you connected the batteries in parallel or did the heat build up over time? If over time, then you may have the rare case of a shorted battery. Actually... The high current flow from the good battery to the dead battery could cause such a short.
But in either case you would have measured very low voltage on the dead/weak battery separately and this is a signal to not connect them in parallel.
You basically connected a 800Amp charger to a dead battery and there was no limit to the charging current.
mgarland48, do you want to add a 2nd battery because you are camping without shore power or "just for insurance"? If, like me, you never camp without an electrical connection, the SRM-24 will probably be adequate. Use it until it dies and then get a new true deep cycle that will last longer than the Dual Purpose Interstate. I bought my TT used and the dealer put the same Interstate SRM-24 battery on it. When that battery dies I'll pickup a decent Deka Group 31 deep cycle from the factory about 20 minutes away.
Wow, great comments from everyone! Super job guys, I do so appreciate the learning experience. My friend just dropped off the new battery for me. I have attached a pic of the Interstate I got with the camper and the new Energizer. The specs seem almost identical except for the MCA. Sams Club was clearing them out as they are not going to carry the Energizer any longer, that's why it was inexpensive $67 (inc core charge). My friend said one guy bought over 100 6V golf cart batteries just before he arrived!
Like I said, the Interstate came with the camper new, not used, in late Oct 2016. We only did maybe 3 days dry camping. I checked the voltage when I brought it home from storage and it's right up there, perfect. I'm getting another one to run in parallel only because I want that assurance while I'm learning (I'm new) to dry camping (which I want to do MUCH more of, rather than pay so much at RV campgrounds). I also purchased a trickle charge cord to run from the bedside out to the batteries for when we are plugged in. My fear (maybe unfounded) is ruining a battery by draining it below 60%. I do have a portable generator as well, which I can use in a pinch.
Thanks Herk7769! Great explanation. It seems what I need to get are batteries with a similar RC value to get equivalent Amp Hour based batteries.
Actually RC is only used to describe Dual Purpose Marine type batteries (that have plates purposely built to fall somewhere between a starting battery and a true deep cycle storage battery). They are the worst of both worlds. They are marginal performers for starting engines and have pretty crappy capacity for longevity vs their weight.
If you want the best bang for your buck in an RV "trailer only" battery you need a deep cycle battery rated in Amp Hours.
Matched pair with same Date is best bet.
Even then one will go bad before the other.
If wired properly (current path resistance into and out of each battery is the same - see diagram) they should age together gracefully.
If wired with both charge/discharge wires on the same battery or different length interconnecting cables, the different inter-battery resistance will cause one battery to work harder than the other and fail first. The lower resistance battery will provide more of the load and recharge faster than the "slacker" on the battery with higher resistance.
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