2 deep cycle batteries and an inverter: run in parallel or one at a time?

Suppose I’m looking at a power outage and have an inverter and 2 deep cycle batteries. To maximize runtime, would it matter if I hooked the batteries up parallel or should I just run one until it’s discharged and then hook up the other one? I realize that from a practical consideration, I should run them one at a time so that I could theoretically charge one (if I can find a source) while using the other. Aside from that, does it make a difference?

Assuming identical batteries and load, running in parallel should last twice as long.

One possible consideration, running in parallel will allow you to pull more current than just one, assuming your inverter can handle it.

This isn’t true. The two batteries contain the same amount of energy regardless of whether you use them one-at-a-time or in parallel.

I meant twice as long as one battery by itself. Then yes, another battery by itself would equal the same as two. We’re saying the same thing, but I admit i was less than clear with my first response.

Use them in sequence. That way you’ll have to minimize the current draw. If you connect them in parallel you’ll just plug in more stuff and drain them both faster.

Depending upon specifics of the situation it could matter a bit.

Batteries of most kinds have different effective capacities depending upon the current drawn. Typically the higher the current draw, the lower the capacity. So in that case you may see some increase in available energy if you parallel them. If you are running a couple of lights and a radio, no big deal. But it could get to be an issue.

If the batteries are not identical (which could simply be because one has developed a fault, even if they were identical make and model bought at the same time) you can get to the situation where when in parallel one goes flat before the other and the good battery starts to try to charge the dead one. You just lose even more available power.

So, like most things in life - it depends.

The consensus opinion in the industry is that in general, it is better to charge and discharge the batteries seperately, not in parallel.

Batteries are never identical, so when you connect two in parallel, you will always get an equalisation current flowing from one to the other. Since charge and discharge are never 100% efficient you will always loose power when you do so.

Of course, if you charged them in parrallel, and kept them connected, you wouldn’t get that equalisation current at the moment of connection. Instead, you’d get two batteries slightly unequally charged (but at the same voltage). The unequal charge and discharge would mean that one would fail faster than the other.

Of course, if you discharge them one at a time, you might completely discharge one, and then have the power come back on. Deep cycling the battery wears it out: you would have been better off using both in parallel.

Confused? Well, yes.

The consensus is that in general, when you have a choice, it’s better to put all your lead-acid cells in series (which is the way a lead-acid battery is constructed), and charge and discharge them that way. If you don’t have that option, well it will probably be years before the batteries wear out anyway.

If you can find a 24volt inverter it will probably be more efficient too. So series makes a lot of sense.

A “battery” is a number of cells wired in series. By connecting more batteries in series you are really just making a bigger battery so to speak.

Trucks (in Europe at least) run on 24 volts and usually have two 12v 120 amp/hour batteries wired in series. This means that your local truck parts supplier will stock a good range of 24 volt equipment like kettles and microwaves.

This gives me some food for thought. I don’t actually have 2 deep cycle batteries, I have one and have considered buying another. If I go with the “run them in series” idea, is it as important that they be “identical”? IE, could I use a deep cycle and a car battery in series without too much ill effect? Of course, that means that I’d have to buy another inverter as well.

A regular car battery won’t last nearly as long as a deep-discharge battery under the type of use you intend for it. Here is a web site that compares the life of a starter battery (e.g. a car battery) vs. a deep-discharge battery for different levels of discharge. A deep-discharge battery can take being fully discharged 150 to 200 times, while a car battery can only take this 12 to 15 times. A deep-discharge battery can take a 30% discharge over 1000 times, while a car battery can take this only 130 to 150 times. You’re really better off using a deep-discharge battery for your application.

If you run two mismatched batteries in series, one goes dead first and then the other one wastes energy trying to charge it.

What input voltage does your inverter require? Most of them that I know of need a specific voltage in order to run. In other words, if the inverter is a 12 volt inverter, it needs that input voltage. To much higher or lower and it won’t run.

I have an 12 volt inverter system powering a cabin, and it needs the input voltage to be basically between 11.5 volts and 14.5 volts - it will shut off if the voltage is out of this range.

For instance, if you have a 12 volt inverter, and two six volt batteries, you need to hook up the batteries in series to get the 12 volts. If you have two 12 volt batteries, you need to hook them up in parallel so that the voltage is still 12 volts.

My system uses six 6 volt deep cycle golf cart batteries. So I have to to hook each two of them in series to get twelve volts, and then hook up each two battery 12 volt cell in parallel to keep the voltage at 12 volts.

In other words, you don’t have very much maneuvering room here. The batteries voltage and the inverter voltage have to match.

Thanks Dopers. It sounds like just buying another deep cycle and running it after the first one conks out would be the safest and easiest choice.

You’d be better off switching batteries before they discharge completely. They’ll last longer that way.

Point taken.

safest and easiest, maybe. But it won’t maximize run time.

The capacity of a deep cycle battery is measured in Amp-hours but you get different answers depending on how fast you try to draw the current. If you take a battery which is rated at 120 Amp-hours, theoretically that means you could draw 6 Amps for 20 hours, or you could draw 60 Amps for 2 hours, which is the same amount of energy. But in the reality, that does not happen. If you tried drawing 60 Amps, you’d find that the battery would run down in less than 2 hours. The reason for this is the fact that lead-acid batteries are more efficient when you are drawing a smaller current.

For example, you might test a 12V battery and find that it has the following characteristics:
Drawing 4 Amps, it lasts for 34.1 hours.
At 5 Amps, it lasts for 25.2 hours.
At 6 Amps, it lasts 20 hours.
At 8 Amps, it lasts 14.3 hours.
At 12 Amps, it lasts 8.9 hours.
The measurement in the middle of that chart is called the C/20 rare, how much current you can draw for 20 hours. The manufacturer would say this battery has a capacity of 120 Ah.

But notice that when you draw twice as much current, you get LESS than twice the run time, because the battery is less efficient at that rate.

Now, if you take one of these batteries and hook it up to a load which is drawing 12 Amps, it will last 8.9 hours, and then you could disconnect it and hook up another identical battery and get another 8.9 hours out of that one, total of 17.8 hours.

But, if you had hooked up the two batteries in parallel, then you would only be drawing 6 Amps from each battery, and they’d last 20 hours instead of 17.8 hours.

Assuming the batteries are identical, you maximize your run time by putting them in parallel.

If they are a new matched pair, age and capacity wise, preferably from the same manufacturing lot, then use them in parallel. If they don’t meet that criteria, then use one at a time.

The inverter should connect to the positive of one, and the negative of the other, so that both connect to the inverter by the same total length of wire. Alternatively, each should have it’s own identical set of leads to the inverter.

Batteries have internal resistance, and drawing half the current from each will result in 1/4the power lost in each battery (so internal resistance loss is halved.) In general batteries will be happier at modest loads. Heavy loads cause a build up of depleted electrolyte between the plates, which increases resistance. Lighter loads allow fresher electrolyte from above and below to diffuse into the plate separator material.

Also if you discharge both to 25% you will get much longer life than if you deplete one to 0% then the second to 50%. For long life of a deep cycle battery, never go below 20% and 50% is much better.

Paralleling two mismatched batteries invites the stronger one to try to charge the weaker one.