First of all, I’ve got to say this one last time: A BMS is generally recommended for a whole pile of reasons, the first of which being safety. A good BMS makes sure you can’t overcharge your battery and keeps it from turning into a dangerous fire hazard. A BMS also makes the charging process much easier.
But if you’re reading this article, you have probably already considered these options and have decided to not use a BMS. In that case, you’re going to need to wire in a balance connector so that you can balance charge your battery when necessary.
Balance charging requires, wait for it…. a balance charger. (Yea, this is tough stuff, I know.)
Cheap balance chargers up to 6s (6 cell groups in series) are readily available. The iMAX B6 charger is the common example, and clones of this common charger are available all over the place for as little as $30.
Balance chargers up to 8s are a bit pricier, and models designed for 10s or 12s can really start to break the bank, depending on your budget. So keep that in mind.
To take advantage of a cheap 6s charger, I’ll build a 4s battery in this example, which will have a nominal voltage of 14.8V and a charge voltage of 16.8V, which is 4.2V per cell.
The normal battery construction is the same as always. Snap together your VRUZEND caps, insert your cells and bolt on the bus bars. You can find other articles and videos on those exact steps here.
After you’ve got your battery assembled, it’s time to the wiring. Without a BMS, you’ll only have two thick gauge wires on your battery: one on the positive terminal and one on the negative terminal These will serve as both your charging AND your discharging connections.
Next, you’ll wire in your balance connector. The best way to find a balance connector is to look for a balance wire extension cable and just cut off the male-pin connector. Notice that there is one more wire than the number of cells that the cable is intended for. This is a 4s balance wire extension cable, so it has 5 wires. You’ll see why in a minute.
Next, find the red wire (or the wire that is intended to the be highest cell number wire if your wires are different colors) and connect it to the same location as your positive discharge wire. This will always be the positive end of the highest numbered cell in your pack. In my 4s pack, this is the positive end of my 4th cell group. I just slide it under the bus bar and tightened the nut, clamping the wire against the bus bar.
Now take the next wire down from the red wire, and connect it to the next cell group down’s positive terminal. This is likely on the other side of your battery. Continue moving down the balance connector wires, connecting each successive wire to the next lower cell group’s positive terminal. The second to last wire should connect to your first cell group’s positive terminal. The last wire will finally connect to the first cell groups negative terminal, which is the same location as your packs main discharge cable. That’s why you’ve got one more wire than cell groups – because the first cell group has a wire on the positive AND negative terminal.
Balance charging without a BMS
To balance charge, you’ll plug your discharge/charge wires into your balance charger (you may have to use one of the adapters that comes with your charger). Then you’ll plug your balance wire connector into the appropriate spot on your charger.
Make sure to read your charger’s instruction manual carefully to select the proper balancing program for your battery. You want to match both the chemistry of the battery (usually marked li-ion or li-po in the charger’s settings) and the charge voltage, which is 4.2V for most li-ion cells. You also want to make sure you select the proper number of cells so that you don’t overcharge your battery.
If everything checks out on your charger, begin the charging process. Balance charging isn’t always necessary if you are using good quality cells. Most batteries will stay fairly well in balance after a few discharge cycles. You’ll need to check your cell groups to make sure they are staying fairly well balanced during discharging, and always balance charge if you see that the cells are becoming unbalanced. If you aren’t balance charging every time though, you can bulk charge.
Bulk charging without a BMS
Bulk charging is basically the same as charging a battery with a BMS, except that there is no BMS to watch the process. When lithium battery fires happen during charging, its usually because someone was bulk charging without a BMS and made a stupid mistake. Always perform bulk charging carefully when not using a BMS.
Bulk charging means that you aren’t balancing each cell group like with balance charging – instead you’re just charging the whole pack up together to a certain voltage, balance be damned. This is ok as long as two things don’t happen: 1) No single cell in the pack goes over the maximum voltage it is rated for, usually 4.2V for li-ion cells, and 2) The pack voltage shouldn’t go over the total proper voltage, which is essentially the # of cells in series multiplied by 4.2V for li-ion cells, and which basically means that the first situation has happened.
To make sure that no cell goes over it’s maximum voltage, bulk charging is normally done to a lower target voltage. For example, instead of trying to charge the pack to 4.2V x # cells in series, consider charging to 4.1V or 4.15V. For a 36V battery with 10 cells in series, that would mean charging to 41V or 41.5V instead of 42V. This adds a level of safety by giving you a buffer for a few cells to overcharge. Without balancing, cells being bulk charged will each charge to a slightly different voltage. If you charged to 42V in a 36V li-ion battery, some cells would reach 4.2V, but others might reach 4.18V, meaning others would have to reach 4.22V to allow the entire pack to reach 42V total. This isn’t terribly unbalanced, but as the imbalance grows, some cells could being reaching 4.3V or higher, which is a very dangerous situation.
This is why I almost never bulk charge without a BMS. If I’m not using a BMS, I always try to balance charge. And if I do bulk charge for a few cycles, I aim for a lower target voltage and I use a cell checker to watch my cell groups and ensure none are overcharging.
Charging safety tips
No matter what type of charging you are doing, there are some important safety tips that you should always follow:
- Never leave a battery charging unattended. Charging in the other room is generally fine. Charging at home while you go out to dinner is not a good idea. The chance of a fire is rare, but it’s not zero. So you should always be around just in case.
- For the same reason, charging overnight while you sleep is not a great idea, unless you are charging in an area where nothing bad would happen. Again, I don’t want you to feel paranoid, but because the chance of a battery fire with any lithium battery is a tiny bit above zero, you should always consider it a possibility. If you need to charge while you’re asleep, a good method is to put the battery in an area where are fire wouldn’t catch anything else on fire. If you have a back porch made of concrete, that could be a good option. Others put their battery in the barbecue grill or oven, which are both areas meant to contain a fire.
- Unplug a battery when it is finished charging. When your battery is finished charging, unplug it. Generally it’s ok to leave it plugged into the charger, assuming both the battery and charger are in good working order. But a charger malfunction could occur at any time, especially in cheap chargers. A charger malfunction could, in rare cases, result in a battery overcharging and catching on fire. Avoid this scenario by disconnecting your charger after your battery has finished charging.
- These tips aren’t meant to scare you, but it is important to understand that even though the odds of a battery fire are incredibly rare, they should still be considered and prepared for.
|About The Author|
|Micah Toll is a mechanical engineer, lithium battery builder and ebike educator. He’s written multiple books including DIY Lithium Batteries (an Amazon #1 Bestseller!) and The Ultimate DIY Ebike Guide (an Amazon #2 Bestseller!). When he’s not tooting around Tel Aviv or Florida on his ebikes, you can probably find him reading, writing, running or vegging out on the couch.|