6 Batteries in Parallel - Good Idea or Bad? Optimas

[solecollector]

I purchased a Lagoon 440 with a nice set of house batteries - the only problem - one of the blue tops (the bigger more powerful ones which means more expensive to replace of course) is dead - won't hold even a partial charge. So when I went to NAPA to replace it they noticed the top wasn't flat - it had big bumps on it. I told him my setup - 6 all hooked together in parallel. When I went to install the bad one - I noticed the one next to it was warm and it had the "bumps". All the others didn't (and this is in the middle of the pack - right next to the last one that went bad).

So I think the setup is the charger goes from one side on out - not positive on one side with the negative on the other. So I assume the first in line get more juice and it becomes less and less powerful of a charge down the line.

Are these batteries made to go in series like this? Or should I get a couple of 8Ds?

And to make it a little more interesting. There are two 4Ds hooked up to this bank that were OG batteries from 2006. The Optimas are 2.5 years old.

[David M]

Are all six batteries in series? In other words, is the output of the entire bank 12 volts? If that is the case then they are all in parallel with each other. In series and you would find the voltage doubled for the batteries in series.

How many amp-hours are these batteries? It might be simpler and more cost effective to have a few 8-D's rather than having more batteries with fewer amp-hours. The exception might be is if you cannot physically lift an 8-D from its location...they weigh quite a bit and it generally takes 2 people to safely lift one, unless you lift weights at the gym every day.

There is such no thing as the batteries closest to the charger getting more charge unless your battery cables are undersized or your terminals are loose or corroded.

In a bank like this you want all the batteries to be the same so they have the same internal resistance and will charge equally...provided there is nothing wrong with any particular battery. Mixing batteries in the same bank is generally not a good idea.

[S/V_Surya]

I am a little confused. You have 6 batteries connected in parrallel without isolation or means to isolate. If that is true that is a bad idea. One bad battery can ruin the good one(s). By serial charge connection you mean the charger is connected to one battery and loops through the positive terminals. The negative in turn loops through all the others negative terminals.
I have a on my boat a charger that supplies three 15 amp outputs, isolated of course. Two sets of golf cart house batteries that can be combined or isolated. And two set of starting batteries. The charger is on one battery, I have to combine them to charge both. I will have to fix that in the future.

[cdreid]

What he said re series vs paralell. Series increases voltage. paralell amperage. Also you have battery capacity etc.. i wouldnt wire radically different batteries together. imagine theres a big drain and you have two batteries at start that can dump 100 more amps than the ones downstream.. they resist that and get hotter.. different voltages at different charge levels.. just sounds like a recipe for disaster

[ontherocks83]

The most important thing is that all of the batteries are of equal size and approximately equal age. If you have 5 new batteries and one old battery the old battery will weaken or limit the better newer batteries. Isolation is important as if one battery does go bad it can damage the rest of the bank, but that risk is up to you. On my boat I have 2 group 24 batteries hooked up in parallel as a house bank and one group 24 as a starting battery. I do not have my house bank batteries isolated from each other so if one does go down it will harm the other but that is a risk I am willing to take. I have a 2 bank charger that will charge the house bank on one set of leads and the starting battery on another set of leads.
My reasoning for not isolating my house bank from each other is both the batteries cost as much as one 4D with about the same amount of output. so it would be the same price wise if I had one 4d that went bad or 2 group 24s that went bad.

[jeepbluetj]

If the 4Ds are flooded lead-acid, they should not be paralleled to the optimas. They're different technology and charge points. (The optima is more of an AGM than a FLA) They will discharge each other.

And dropping in a brand new battery or two into an older bank will likely increase the rate of self-discharge a bit too.

Personally, I think optimas are a lousy selection for boats. They offer weight savings and vibration resistance over flooded lead acid, but at a significantly higher price and lower capacity. They're great for jeeps though.

I'd replace with flooded 8D's or even 4 golf cart batteries. (wired series-paralell). If you want zero maintenance go AGM. Your charger should be set correctly for battery type too - AGM, Gel, or flooded all have different change algorithms.

[Andina Marie]

Originally Posted by ontherocks83

The most important thing is that all of the batteries are of equal size and approximately equal age. If you have 5 new batteries and one old battery the old battery will weaken or limit the better newer batteries..

Oh how the rumors persist

How will an old battery "Weaken or limit the newer batteries"? True the total capacity available will be less that if it were a new battery but depending on how old it is you may as well use it until it no longer adds significant capacity.

There is no reason for batteries in parallel to be of equal size and age, that only applies to batteries in series. Each battery in parallel contributes to the load, and accepts charge, exactly in proportion to its capacity. It is a common misconception that batteries in a parallel circuit split the load and charge current equally. Nothing could be further from the fact. The output capacity of the bank is not limited by the weakest battery (like it is when in series), the weakest battery contributes less but still contributes.

Isolation is important as if one battery does go bad it can damage the rest of the bank, but that risk is up to you. On my boat I have 2 group 24 batteries hooked up in parallel as a house bank and one group 24 as a starting battery. I do not have my house bank batteries isolated from each other so if one does go down it will harm the other but that is a risk I am willing to take.

If a cell dies in one of the batteries in parallel it doesn't DAMAGE the rest of the bank, it just discharges them. The charger will only get up to about 12.5 volts and the bad battery will get hot. Replace the bad one, recharge, and you are now back in business.

Technical discussion of cell failure for those interested.

It is an interesting fact that battery life of batteries in parallel can be increased over a single large battery of the same capacity as the bank. Here is how it works. A cell goes bad when due to charge cycles the metal in the plates builds up unevenly until the "hill" on one plate touches the opposite plate. When this happens the cell discharges itself through the short circuit and battery voltage drops by 2 volts. If it is just a small short, the discharge current of the cell will burn off the short and you never even knew it happened. If it gets so bad, or the charge in the cell is insufficient, and it can't melt it off then you have a bad battery.

However if you have multiple batteries in parallel, when the cell goes bad and the voltage on that battery drops by 2 volts, the other batteries can deliver hundreds of amps to the bad battery which will be dissipated in the short circuit in the bad cell. This gives you 10, or 100 times the current to melt the short circuit and often restore the cell. If it doesn't then all the batteries will be 2 volts less but not ruined, just in need of a good charge when the bad battery is replaced.

[ontherocks83]

If you put 2 batteries together of different sizes/ages they will have different capacities. When charging 2 batteries in one bank with different capacities they will need to be charged at different levels. Either the weak one will not get enough power or the good one will get over charged and you can get excessive gassing which then can lead to dry cells and a damaged battery.

If one battery is at say 10.5 volts when it stands alone and you connect it to another battery that is at 12.7 volts and then you connect it to a charger it is not going to properly charge the weak battery because it thinks the overall voltage of both batteries is the same when one is significantly weaker. At the same time when the good battery doesn't really need to be charged it will continue getting charged until the weaker battery is at an acceptable level which again can lead to excessive gassing from the good battery.

Now lets say you have a battery bank where one battery has a bad cell and drops the voltage by 2 volts across the bank. Yes you can charge them back up, but if the voltage has dropped below 10.5-11 volts then you are damaging the batteries. Even deep cycle marine batteries can only be discharged that low a small number of times before they have significant damage. Now I'm not saying the batteries wont work but they wont have nearly the capacity they used to. And to prove the point that is why it is common knowledge to try to recharge your batteries before they get below 12 volts to extend the life of your batteries by as much as possible.

There is a difference between what will work and what is best for your investment (usually costly when you get into bigger batteries) and more crucially for your power system which can be your only life line when you are on the water.

[Stu Jackson]

8 D batteries.

UGH!

[CarlF]

My understanding is that it is best practice in such banks to connect the positive battery cable and negative battery cable to batteries at the opposite ends of the set.

The reason is that the batteries cables between the batteries have resistance and canl cause the batteries to discharge somewhat unevenly. If the battery cables are connected to the same battery, that battery and it's closest neighbors will be cycled deeper and will fail sooner.

Connecting the cables at opposite ends evens this out somewhat (although I guess not perfectly)

Carl

[ontherocks83]

Originally Posted by CarlF

My understanding is that it is best practice in such banks to connect the positive battery cable and negative battery cable to batteries at the opposite ends of the set.

The reason is that the batteries cables between the batteries have resistance and canl cause the batteries to discharge somewhat unevenly. If the battery cables are connected to the same battery, that battery and it's closest neighbors will be cycled deeper and will fail sooner.

Connecting the cables at opposite ends evens this out somewhat (although I guess not perfectly)

Carl

From the studying I've done my findings are that yes this is a true statement but the negative effects to having them connected on the same battery is minimal. Again falls under the category of it cant hurt to have it on opposite ends, but I would not be overly stressed if you have them hooked to the same side

[Andina Marie]

Originally Posted by ontherocks83

If you put 2 batteries together of different sizes/ages they will have different capacities. When charging 2 batteries in one bank with different capacities they will need to be charged at different levels. Either the weak one will not get enough power or the good one will get over charged and you can get excessive gassing which then can lead to dry cells and a damaged battery.

Sorry, that is totally wrong.
How can one battery be overcharged while the other doesn't get enough charge? THEY ARE AT THE SAME VOLTAGE. If you were overcharging one then its voltage would have to be too high but if the charger is working that won't happen. You can't over charge one without overcharging the other. If they get to a fully charged voltage, then both are fully charged to exactly the same voltage.

If one battery is at say 10.5 volts when it stands alone and you connect it to another battery that is at 12.7 volts and then you connect it to a charger it is not going to properly charge the weak battery because it thinks the overall voltage of both batteries is the same when one is significantly weaker. At the same time when the good battery doesn't really need to be charged it will continue getting charged until the weaker battery is at an acceptable level which again can lead to excessive gassing from the good battery.

Just a minute, the discussion was about using a bank of 6 batteries in parallel, so there is no way you are connecting one at 10.5 to one at 12.7 as you suggest, they are already AT THE SAME VOLTAGE. However if you did connect two batteries in parallel that were at different voltages, current would flow rapidly from the higher voltage one to the lower and within a few seconds they would be at the same voltage.

Now lets say you have a battery bank where one battery has a bad cell and drops the voltage by 2 volts across the bank. Yes you can charge them back up, but if the voltage has dropped below 10.5-11 volts then you are damaging the batteries. Even deep cycle marine batteries can only be discharged that low a small number of times before they have significant damage. Now I'm not saying the batteries wont work but they wont have nearly the capacity they used to. And to prove the point that is why it is common knowledge to try to recharge your batteries before they get below 12 volts to extend the life of your batteries by as much as possible.

This is true to the extent that a deep, but not fatal, discharge will reduce battery life but getting a bad (shorted) cell is sufficiently rare that one deep discharge in the life of the other batteries will make virtually no difference. Compare that with the cost of replacing a single battery equal to the total capacity when it gets a shorted cell. At least you have incremental replacement costs.

There is a difference between what will work and what is best for your investment (usually costly when you get into bigger batteries) and more crucially for your power system which can be your only life line when you are on the water.

That is debatable. If your single large house battery dies you are stuck without a battery until you can complete your voyage and replace it.
If one battery in your bank of 6 dies you can disconnect it, recharge and run with 5 batteries.
Seems like the latter would give you more "life lines when on the water" than having all your eggs in one basket.


I worked in a telephone exchange with a 3,000 sq ft battery room. The batteries each weighed about 2,000 lbs. We never considered replacing the whole bank if one battery died, you replaced the faulty battery. True some had larger capacity than others, some were new, some were old, but they all contributed to the load.

Don't feel bad, it is a very common mistake, repeated frequently in these forums, to assume if you charge 2 batteries with 40 amps, that 20 amps HAS to go to each battery and overcharge a small one while failing to charge the other. But fortunately the INTERNAL RESISTANCE of the weaker battery is much higher so it takes much less current for a particular voltage so the current divides exactly in proportion to their capacity.

[Andina Marie]

Originally Posted by ontherocks83

From the studying I've done my findings are that yes this is a true statement but the negative effects to having them connected on the same battery is minimal. Again falls under the category of it cant hurt to have it on opposite ends, but I would not be overly stressed if you have them hooked to the same side

Keep in mind too, that as the batteries reach full charge the current drops to zero. As the current drops to zero, the voltage drop on the battery jumpers also drops to zero so at zero or nearly zero they are all at the same voltage within 1/1000 volt.
Bottom line is that normal manufacturing variations in even a matched set of batteries are an order of magnitude larger than the negligible resistance of battery jumpers fed from one end so you are arguing over the socks instead of the shoes.