Noob question about marrying batteries

[JavaJunkie]

I've never used a dual battery mod. Never felt the need for it in my life, but I know that it's important to "marry" the batteries.

I assume this means that you buy two new batteries, you use them together, and you charge them together. These batteries should have the same specs.

Is that correct?

And, why?

What's the difference between getting two freshly charged batteries (same specs) versus having two batteries that you only use together? Does it have to do with that as lithium-ion batteries age the efficiency drops so even if they're listed as the same specs they're not functioning at the same level? Or, is there another reason?

Trying to understand the technical necessity of this.

Appreciate any help. Thanks!

 

[DaveP]

Discharge rates can vary when batteries get older and have been through many charge cycles. Two batteries in parallel can go into reverse discharge when one is at a different charge level than the other, kind of like two air tanks equalizing if they are connected in parallel without one way valves to prevent it.

When batteries go into reverse discharge they can heat up by the sudden charging of the lower voltage battery by the higher charged battery. Most high quality mods will have high amp diode protection to prevent that. The diodes only allow conduction in one direction and won't allow reverse charge to happen. Still, not knowing what's in the circuitry in the box, it's a good idea to match battery pairs and keep them together throughout their life if they are to be used in a parallel battery mod.

 

[TXBiker]

Just don't use old over cycled and mismatched batteries. Honestly you don't know anything about batteries you buy together either. How long have they set on a shelf or warehouse? Did you get one from batteries that have been sitting and another from a fresh shipment etc... I haven't seen any cell matching equipment like I had years back when I matched NiCad's. You don't know exactly what voltage, and amperage the battery is capable of, or the time from full charge to low voltage cut off, or internal resistance. Batteries change through out their life cycle and no two are ever the same. The two batteries that have been in my Sig the past year were used in single battery mods first. There might have been a 7 or 8 cycle difference between the two and no problems to date. If you have a new set keep them together you'll be fine.

 

[Spirometry]

You are trying to match the internal resistance of the batteries. As the batteries age, the internal resistance will creep up. If the internal resistance aren't the same, one battery will provide more amps than the other.

The problem arises when you are near the CDR of the cells. Say you have a high watt build and some 20 amp cells. If your cells are grossly mismatched, it is possible to draw 10 amps from one cell and 30 amps from the other, exceeding the CDR on one of the cells.

By "marrying" the cells, you assume the internal resistance should be close to one another.

 

[JavaJunkie]

Thank you all for your answers. I think I understand now. Makes sense!

 

[DaveP]

The stock answer is to buy batteries in pairs from a reputable vendor and keep them together as a matched pair throughout their life. You won't actually know how "matched" they are because vendors pick two from shelf stock and drop them in a box. One could be older than the other by a significant amount.

If your charger displays actual battery voltage during the charging process you can get an idea of just how matched your batteries are. I tend to watch the voltage display when I insert two batteries that have discharged as a pair in my Smok Koopor 200W.

Both the Efest 35W (over rated and really 20W and sometimes 10W CDR batts depending on the brand that Efest put in the sleeve) and my Samsung 25R's start out in the charger at the same voltage and continue to match as they charge. If I take them out when they are showing half discharge on the mod, they will show the same (equal) voltage when put into my Efest LUC 4 charger.

That's not a laboratory level test, but it's a good indication that they aren't severely mismatched. I'm also a 10W vaper and don't stress my batts, so I'm not worried about exceeding the discharge rate.