Monkey University Reboot: Parallel vs Series batteries.

This is the first installment of the Monkey University Reboot. I won’t have a schedule for these, it’ll be more of a “As I come up with topics to discuss” type of situation. Today’s topic: Parallel vs Series battery configurations.

A lot of devices out, and coming out, are utilizing more than 1 battery. Some are running in Parallel and others in Series. Let’s lay a little ground work.
Parallel configuration means you’ve got all of the negative terminals on one side and all of the positive terminals on the other. I.e. you connect the negative side of all of the batteries to one another and to the ground or negative terminal of your circuit (or mod), and you connect the positive terminals of all of the batteries together and then connect that to the positive terminal of your circuit (or mod).

Series configuration has the batteries in.. well, in series. You have the circuit’s negative connected to one battery’s negative terminal, then you ‘stack’ the batteries. The battery that is connected to the negative terminal will connect it’s positive terminal to the negative terminal of the next battery and that will repeat until you’ve got the number of batteries you want in the chain and then the positive terminal of the last battery connects to the positive terminal of your circuit. Think about a maglight or any other flashlight that uses multiple batteries in a tube stacked on top of each other. This is a series configuration.

*** Important note: Always use identical batteries when using multiple in series OR parallel. It’s important that the batteries are the same make, model, mAh, voltage, etc. It’s best if they are both the same age as well, as in, pair 2 batteries at birth and keep them together.  For the sake of the rest of this post, I will be assuming we are using matching, 3.7v batteries.***

In a parallel configuration you are basically making 1 larger version of the battery by adding more to it. The voltage output will stay the same, 3.7v. However, you will be adding the mAh of all the batteries together. So if you’re using 2000mAh batteries and you put 2 of them in Parallel, you now, for all intents and purposes,  have 1 battery that is 3.7v and 4000mAh.

In series configurations you are going to add the voltage of each additional battery to the available supply. The mAh rating is unchanged. So, 2 3.7v 2000mAh batteries in Series will result in a 7.4v output with 2000mAh.

Discharge ratings on batteries are based on their mAh, roughly (actually, mAh is based on the discharge, but let’s not confuse things). Using a 25amp continuous discharge, 2500mAh batteries as our example, you have a battery that has a “C” rating of 10. To determine your discharge capability you take the C rating of the battery and multiply it by the Amp hours the battery is rated for. So a 2500mAh battery is 2.5Amp hours, * 10C = 25Amp continuous discharge. Now, in parallel you’ve basically created 1 large battery that has 5000mAh, still 3.7v and still 10C, that means we’ve not got a power source that can handle 50Amp continuous discharge. When you go series with the same cells, you’ve got a battery that has a 7.4v output, but still only a 10C rating and still only a 2500mAh rating. That means it is still just a 25Amp battery.

So, which one is best? That’s a very complicated question to answer. In regards to power output, they’re equal. While the Parallel battery might seem to be better due to the 50amp discharge, you have to remember that you’re getting 7.4v out of a series configured battery.

I’m going to use some imaginary batteries here that run at 3.7v perfectly, regardless of load just to keep the math from getting overly complicated.

Let’s push that Parallel set up to it’s max, 50amp. To get a 50amp draw on a 3.7v source, we’re looking at ~.074Ω. yes, point zero seven four. That will output 185W.

Now, let’s push the Series battery to it’s max of 25amp. To get a 25amp draw on a 7.4v source, we’re looking at around a .296Ω coil and… 185W.

Yes, they both do the same max output… and here’s the fun part. If you were to fire both of those devices, the series and the parallel, using these imaginary batteries that run from full to empty at their rated voltage, both batteries would last the same amount of time as well. They are virtually identical as far as output.

It does get a little more complicated than that in real world application, but ultimately if you’re talking about an unregulated situation with a rebuildable atomizer, these two configurations are completely even as far as vape time, output, etc.

Why use one over the other?

In some cases, people just want more battery life in a device, so they slap another battery in there in parallel and double their mAh and thereby double the length of time the battery lasts before it’s “dead”. In other cases, people want more power.

In an unregulated device, going parallel or series ultimately doesn’t matter. However, due to the fact that our ‘standard’ is based on 3.7v, people tend to build coils around that voltage so going parallel gives them a bigger tank and thereby a bigger discharge capability and they can get the higher wattages they want.

In regulated devices we see a lot of series configurations. The newest SX chips as an example will put out 60w with a single battery or 120w with a series battery set up. Based on the previous information, why don’t they just use parallel batteries since you can get the same power? The answer involves getting rid of our magic battery that runs at a perfect nominal voltage regardless of the load applied.

In battery testing, the higher current you draw from the battery the more ‘sag’ you see or voltage drop as it’s called in our industry. This means, you might be seeing 4.2 on an unloaded battery being tested by a multimeter, but if you hook it up to a .5ohm atomizer and read the voltage on the circuit, you could be seeing 3.8v, as the battery depletes it’s stores of energy that will slowly decline to a certain point until it nose dives. On a freshly charged battery, good quality battery that’s relatively new, you’ll see ~3.9v with a .5ohm load. After the first hour or so, that’ll drop down to 3.7v and float there for quite a while. When you get towards the lower end of the batteries charge, you’ll see a dramatic drop off and it’ll rapidly go from 3.7v down to 3.0 and below when being fired. The current draw becomes more difficult with lower resistance loads, so if you ran a .1 on that same battery you’re going to see it drop down to 3.5-3.6 even when fully charged and fresh and it will rapidly degrade to the lower 3.X’s while being fired. This voltage sag is due to the batteries inability to crank out the energy fast enough due to the chemical and mechanical limitations of our cells. More current equals more work for the battery, lower resistances or higher wattage outputs always equals more amps being drawn from the cell.

With a regulated device, stability of source voltage is important, when you start pushing into that 100+ watt range, you need to run series configuration so that the batteries aren’t being taxed. Raising the voltage by stacking them has a lower impact on the actual process that outputs energy from the cells which means they will sag less at comparable power outputs. Hence why the ultra high wattage devices on the market use Series configurations instead of parallel configurations.

That said, there are a LOT more factors and variables that I’ve not discussed in here and you can lose an entire day just reading articles on the science of lithium based batteries, this was just meant to be a bit of a primer.

Now, last, but most definitely NOT least. Safety. I touched briefly on it in my note about using matched pairs and marrying the batteries, but I’ll expand on that slightly. You want to use matched pairs because the batteries are essentially sharing the load, whether in series or parallel, they will attempt to split the work evenly. If you use unmatched batteries of different capabilities and mAh, they have different characteristics that will make the load balancing uneven and one cell will end up working much harder than the other, but the features of your circuit outside of the battery don’t change. This means you might think you can run X ohms safely and you really cant. One battery will be over taxed, discharge faster and hit a lower voltage earlier which will quickly put it into the ‘over discharged’ realm and that’s one of the more dangerous places for a battery to be. By matching batteries, make, model, age, charge level, you are making sure the work is shared evenly between the two cells. This is important in both series and parallel, but moreso in series. In parallel configurations charge difference isn’t quite as big of a deal because the batteries are connected to one another, even when the circuit is ‘open’ (i.e. you’re not firing the mod) and they will leach off of each other to balance themselves to a certain extent. In series configurations, the circuit is broken when not in use so you don’t get any of that auto balancing.

Charging batteries used in multi-cell set ups is also important. You should charge them the same way and to the same level. I highly recommend a 2 bay charger with independent charging for each bay and a read out so you can see the end voltage on each cell. If one battery is charging up to 4.2 and the other only up to 4.1, you’ve got one cell that’s getting more worn than the other. If you take your batteries out and put them in the charger you might notice one is lower than the other. This isn’t a ‘huge’ deal unless the difference is quite large (i.e. one is 3.5 and the other is 4.1). Being off a little bit is going to happen for a variety of reasons, as long as you never see a huge difference, you’re ok. I recommend rotating the batteries order in the device, both series and parallel. A good way to do that is to mark them, 1 and 2, and everytime you put em in, swap the order. This will make sure the batteries get worn evenly and no one cell is abused too much.

All that said, if you buy 2 batteries together, brand new, and slap em in a multi-cell device, there’s no real guarantee  that the batteries are properly matched. It’s very hard, without expensive test equipment, to make a perfectly matched pair. As long as you stick to the rule of same make/model/age, you’ll be pretty close as the batteries meet certain standards to get the labels they get. (obviously, recognized manufacturers like Samsung, Sony, LG are going to have tighter standards than some ‘other’ brands.). Just monitor the level the batteries are at when they’re put in the charger, and make sure you don’t see any major discrepancies in charge level or discharged level.

Charging batteries IN a device can get complicated. Parallel battery packs, for the most part, can safely be charged by any regular old on-board charger you’ll find in a device. Because they balance themselves to some level and they are, for all intents and purposes, one big battery when configured in parallel it’s not a huge risk. My personal recommendation though, if you CAN change the batteries in your device, at least rotate them between charges if you’re using the onboard charger. Series batteries are where it gets more important to pay attention to how you charge them. Because they don’t have that auto leveling function found in parallel set ups, you really need to balance charge them. The pack is running at 7.4v, you might think you can just use a 7.4v charger and hook it up like a big ol battery, the issue here is that you’ll be charging one cell THROUGH the other cell and that’s going to wear out that lead cell very quickly, it will also not take the individual cell’s charged status into account. You see reference to “balance chargers” in the RC world a lot, and this is what they are for. Packs that are all assembled will have leads run to each individual cell for balancing them while being charged instead of just hooking up to one cathode and anode and letting god figure out the rest. I highly recommend, with any series battery device that you either charge the batteries individually or do some serious research on the charging method used by the onboard charger if it has one.

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Monkey University: Parallel and Series Batteries by SteamMonkey LLC is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
Based on a work at http://blog.steammonkey.com/?p=320.