Question about 18650 batteries and charging

Hi all, first post here. I'll dive straight in with a technical question

I have a Hope Vision dual LED for my off road mountain biking. Its fine, but the 7.4V 2.5ah battery life is a little bit on the low side. I therefore have bought 2 18650 batteries with a combined 7.4V and 4.8ah.

The Hope light already uses 18650 batteries. Here is the battery pack:



This pack is charged with this:



The batteries I have bought are:



And this is the charger I have bought for them:



What I'd like to know is, are these batteries charged on a trickle-charge where the current slowly drops as the battery voltage rises, or is there some kind of electronic control going on? Its a consideration as I may (for convenience) choose to put a spare connector on the 2 new batteries which matches both the Hope charger and the Hope light. Otherwise I'll have to go out and buy several new connectors and make a couple of leads up.

Your new 2 UltraFire cells have 7.4V and 2.4Ah nominal. The original battery back should have more capacity if the cells are in the same condition. You'll have to open up the original pack, I' imagine there is a circuit in the top of the pack that will prevent overcharge and overdischarge for both cells together. I don't think it's practical to use a pair of those 18650 cells unless you make a battery holder with the proper connector.

I don't think it's practical to use a pair of protected cells in a stick pack because of the way those protection circuits work they require individual attention.

Parrot of Doom said:

Hi all, first post here. I'll dive straight in with a technical question

I have a Hope Vision dual LED for my off road mountain biking. Its fine, but the 7.4V 2.5ah battery life is a little bit on the low side. I therefore have bought 2 18650 batteries with a combined 7.4V and 4.8ah.

Click to expand...

:welcome:
Actually, you can connect the batteries in parallel (so they're at the same voltage, and each supply half the current; 3.7V, 4.8Ah) or in series (they each supply full current, and half the voltage: 7.4V, 2.4Ah). To make a 7.4V 4.8Ah pack, you'd need 2x 4.8Ah (D-size or so) cells in series, or 4x 18650s like you have, in a 2s2p arrangement. Not sure if this has to fit inside your light; if so, you may not be able to make it any bigger.

What I'd like to know is, are these batteries charged on a trickle-charge where the current slowly drops as the battery voltage rises, or is there some kind of electronic control going on? Its a consideration as I may (for convenience) choose to put a spare connector on the 2 new batteries which matches both the Hope charger and the Hope light. Otherwise I'll have to go out and buy several new connectors and make a couple of leads up.

Click to expand...

LiCo cells are usually charged with a 4.2V CC-CV cycle; trickle charging is harmful to them, as they can't accept further current (even low levels) without overcharging.

I believe the DSD charger doesn't do a proper CC-CV cycle, but some close-enough cycle (something cheaper to make...); I'm pretty sure the cells must be separate for it to work (no adapting it to charge a pack).

The 2-cell series charger that came with the light should do fine for charging the batteries as a pack, as long as the batteries are matched in condition and charge level, and stay that way. I'd charge them in the DSD, so they're both at full capacity, before making up the pack. That will be as good as the stock setup, anyway. (A series charger isn't a real good approach, because imbalance between the cells can cause problems and isn't corrected during charging, but it's not that bad for 2-cell packs.) A 2S2P pack would also be fine with the same charger.

Not sure how you plan to assemble the cells into a pack -- soldering on the ends of Li-ion cells is really dangerous unless you know what you're doing (in which case it's only a calculated risk that many avoid anyway), so you should probably either spot-weld tabs on, or use a holder (space permitting). Turboferret makes some nice 18650 holders that might work out well, either a 2s twin holder for the cells you have, or a 2s2p quad holder if you get two more for the capacity you had initially planned on. The 2s might still fit in the space for your battery pack, if that's an issue, but it's probably too big. Maybe put a long cable on it, and mount the pack externally.

Oh, one last thought: Whatever you do, you'll likely need to add a 2-cell protection circuit, since those look like unprotected cells. Otherwise you will risk serious problems, up to and including explosions while charging, from damage due to overcharge, overdischarge, or inadvertent short-circuits. Running cells in a pack only increases these risks...

Hello again, and thanks for the replies This is a fascinating place.

I've had experience with cell imbalance for a while. I'm a TV cameraman by trade and am well used to working with a variety of batteries. We used to use Nicad BP-90s before moving onto PAGs, and now the latest batteries are Li-On V-locks like these:

http://www.proav.co.uk/Batteries-Camcorder-Power/V-Lock-Batteries/sc599_c586.aspx

Before PAG started making intelligent chargers the old batteries would regularly only charge to less than 100%, obviously with some cells containing a higher proportional charge than their neighbours. Right now I'm using a SWIT NP-1 lithium battery on my sound kit. This is however the first time I've ever DIY'd a battery pack, and I'm doing it half from general curiosity, and half because if I can do it myself I don't want to pay anyone else

Anyhow, I already have the 2 18650 cells in a holder, and have connected a wire to them. The Hope light I have was supplied with an extension cable so I merely cut one end from that and connected things up. The advantage for me is that the cable connector is waterproof and moulded. Here's a pic (please ignore the bodgy wiring, that'll be tidied up).



(If you want to know where to get the holders I can dig that out for you, he does single cell and quad-cell versions).

I think the existing Hope pack I have means that each cell is 3.7V and 1.3ah, wired in series to produce 7.4V and 2.6ah - or does capacity not sum in that way? The voltage supplied to the light is certainly 7.4V so they're wired in series, which is how I have wired the pack I've made up. I've already tested the DIY battery, the light is quite happy but I haven't done it for extended periods as the light will get hot, it needs to be run with the wind in its hair

The main reason for all this is if the Hope charger can charge my DIY pack, I can completely encase the DIY pack in rubber and make it fully waterproof - this is the same as the Hope battery pack. I'll can then off-road at night for 4-5 hours, instead of 2, on full power.

Parrot of Doom said:

Anyhow, I already have the 2 18650 cells in a holder, and have connected a wire to them. The Hope light I have was supplied with an extension cable so I merely cut one end from that and connected things up. The advantage for me is that the cable connector is waterproof and moulded. Here's a pic (please ignore the bodgy wiring, that'll be tidied up).

(If you want to know where to get the holders I can dig that out for you, he does single cell and quad-cell versions).

Click to expand...

Looks great; I think that holder looks like one of turboferret's that I linked to above.

I think the existing Hope pack I have means that each cell is 3.7V and 1.3ah, wired in series to produce 7.4V and 2.6ah - or does capacity not sum in that way?

Click to expand...

The pack contains two 3.7V 2.6Ah cells. Capacity does not sum that way; charge capacity in Ah is just current (which sums in parallel, and stays the same in series) multiplied by time, which isn't affected by the number of cells. If you measure energy capacity in Wh, then it does sum that way -- parallel doubles current, series doubles voltage, and either one of those doubles power.

So each of your cells is 3.7V, 2.4Ah, so the energy per cell is 3.7V * 2.4Ah = 8.9Wh; 2 cells will be 2 * 8.9Wh = 19.8Wh, and can be connected either series (7.4V * 2.4Ah = 19.8Wh) or parallel (3.7V * 4.8Ah = 19.8Wh).

The main reason for all this is if the Hope charger can charge my DIY pack, I can completely encase the DIY pack in rubber and make it fully waterproof - this is the same as the Hope battery pack. I'll can then off-road at night for 4-5 hours, instead of 2, on full power.

Click to expand...

That'll work fine, except you'll only get 2 hours, because you have about the same capacity. If the old pack is still good, of course, you could swap them out for 4 hours total. But the Hope charger should be fine with a 4-cell 2s2p pack, so that's what I'd do. I'd be tempted just to parallel the new and old packs to make 2s2p, but if your old pack has significant cycles on it, you might be dooming the combined pack to fail early as that stack gives out.

A few things to consider.

If you are intending on using the charger included with your bike light, you are going to need to determine the charge method and termination type that the charger uses before attempting this. If your ultrafire cells are unprotected, and the charger uses the PCB of the pack to terminate a CC-only charge method, then you will burn your house down. This charging method is very popular in "budget" series li-ion packs as it eliminates the need for an expensive balance-tap/balancing charger.

$5 bet on the pack you already had having better runtime than the new ultrafire cells. UF cells are known as being flakey/lousy. Many people have tested them and they come up very short of rated capacity most of the time.

The DSD charger uses a slow Constant-Current charge to ~4.25V and then terminates the charge. When they are working correctly (watch out for duds) they are some of the safer cheap chargers out there if used correctly as they use a very similar charge method as what's found in most cell phones. It's no coincidence that the included power supply is a cell phone power supply. Keep in mind that the DSD wires any cells you install into it in parallel, so it's important that any 2 cells you install into it be in a similar state of charge. Installing one near-full charged cell with a mostly-discharged cell would result in a charge rate on the discharged cell that would likely be above the maximum recommended as the charged cell tries to balance with the discharged cell.

Conservation of energy: Why would putting 2 cells together result in quadruple the stored energy as a single cell?

-Eric

Okay then, so lets for the time being discount using the Hope charger. It's no problem, I can waterproof the pack well enough using a tied-off inner tube that can be released easily enough to remove the cells and place them on charge.

Really all I need is a cheap and cheerful way of extending my night-rides, so I'll see how things go. I bought the charger to charge some CR123s that I also bought for powering radio microphones (damn site cheaper than buying 6 new ones all the time).

I think I'll charge those cells with care. I certainly won't leave them unattended.

If they are unprotected cells, you're also going to need to determine how the bike-light terminates the discharge to protect the cells from over-discharge. If the bike-light relies on the packs PCB, then you will need to employ your own means of terminating the discharge to prevent permanent damage to the cells.

Well the light goes into a low-power mode when battery voltage is low enough. I had presumed that the light does this itself, so I'll run it through a cycle and see what happens.

Do you guys have a recommended protection circuit I could use for both, or each cell?