Battery Pack Building Techniques

Safety First.  Before we even begin, it is imperative to grasp the importance of workplace safety.  Shorted lithium batteries can erupt into fireballs that are nearly impossible to extinguish.  There is a reason lithium batteries have major restrictions on how they may be transported.  You absolutely MUST respect the danger of fire and explosion with lithium batteries, particularly the lithium-ion chemistries that contain cobalt. (LiFePO4 cells are safer, but can still short and cause fires).

Work Area Security: Secure your work area so that children and animals, and even other adults can not touch or shift/move cells and packs before you’ve completed your build.  Just because you are personally careful doesn’t mean that visitors to your worksite will be equally careful.

Eyes and Hands:  Battery building requires two essential pieces of safety equipment:  Safety Goggles and Rubber Gloves.

Essential Safety Equipment

Welding batteries together can result in sparks of hot metal.  Not only can they put your eyes out, but they can land on flammable material (such as carpet, fuels, other batteries) and cause uncontrollable fires.

We always keep a pack of orange rubber gloves from Home Depot available in our shop. Rubber gloves will help you keep from getting shocked, and from shorting out delicate equipment such as BMS circuit boards and other goodies. Keep extra gloves and goggles readily available in your work area.

Sparks, metal shards, wire trimmings: Remove scraps of metal from nickel strips or wire clippings as you work.  In fact, take the time to pick up and dispose of each one as you create it.  We’ve even seen sparks from a spot welder flying through the air and down onto carpet.

DO NOT underestimate the danger of working with lithium batteries.  Respect your craft and your battery-building skills, and respect the life and property around you.  Be safe or find something else to do.

Hot glue cells together. Let dry, flip over, and glue again.

Let’s Get It On:  We build 4S packs for many of our small projects.  When combined with a cheap voltage regulator, this pack can provide power for a multitude of projects across a wide range of voltages.  Add a few parallel stacks, and you can boost the amperage up to run all sorts of equipment.  Plus they are easy to charge.  We do all our charging from solar panels connected to a cc/cv charge controller.

CC/CV stands for Constant Current / Constant Voltage.  Lithium-based batteries need to be charged with these specialized charge controllers.  They charge on a curve that initially keeps current constant in the initial stages of charging while voltage changes as the battery begins to fill.  As the battery starts getting closer to a full charge, the CV portion of the charge-curve takes over, and voltage stays constant while current slows down until there is no more current.  Every CC/CV charger will have datasheets describing the full charging curve.

Inexpensive Spot Welder in our shop.

Spot Welder:  An inexpensive spot welder is better than soldering cells together.  Soldering puts too much heat into your cells, dramatically cutting down their lifespan.  These can be had for a few hundred bucks.  They aren’t very strong though.  We wrap the knob all the way up, and keep the pulse setting on 6 pulses per weld.  If you are so inclined, we’ve seen DIY welders made out of giant capacitors, but we’ve also read stories of holes being blown into cells, and even cell explosions.

Our welder includes a professional-grade soldering iron.  We can crank the soldering temperature up, and the unit will chirp away.  As the chirping slows down, it lets us know the iron has reached the designated temperature.  After a quick solder, we spin the knob back down to zero, thus rendering the workspace safer, and also saving electricity.

Spot Welder with Integrated Soldering Iron

Once we set up our piece for the next solder, we dial the temperature back up, and within seconds the iron reaches soldering temp, and we get into it.

For most work we use regular old-fashioned soldering irons that you plug into the wall (or an inverter attached to packs in our case), and wait for the iron to heat up, but when it comes to doing numerous joins on battery packs or intricate circuit boards, we always reach for the integrated iron on our spot welder.

Parallel groups ready for welding.
Pack-Building Pointers:

Plastic Pry-Bars:  Get yourself a set of cheap plastic tools.  We use trim and molding tools from Harbor Freight.  They are only a couple bucks, and we use them when harvesting cells from old packs, and for probing our welds as we build new packs.  Having a set of plastic pry-bars is particularly helpful in battery building, and frankly, I can’t imagine building packs without them.

Keep your favorite plastic pry-bar handy, and use it throughout your build.  Gently pry your welds as you work to make sure everything is secure.  In our experience, loose welds account for nearly every failure we’ve found in packs.

Check your welds with a plastic pry-bar.

Capacity Matching: It is important to build your parallel groups with cells of the same or similar capacity.  You can do this by testing the capacity of all your cells with a specialized tester, and selecting cells for your parallel groups based on similar test results. A parallel group is only as good as it’s weakest cell.  This is why builders try to match capacities across cells in a group.  It isn’t truly mandatory, but it will definitely help. If you are using new cells from the same manufacture batch, you won’t need to bother with capacity checking.  If you are combining different manufacture-date cells (or different suppliers, or recycled cells) you may want to take the time to test capacity.

Voltage Matching:  BMS manufacturers tell us that it is critically important to match voltages across all your batteries before connecting the BMS.  We hear over and over that cells should all be within 0.5V of each other.

Parallel groups ready for equalizing.

Equalize Voltages: Industrial-scale battery builders use large “equalizer” boxes which have copper plates on the top and bottom.  Cells are stood in the box, all positive up, and all negatives down, and the top is then put on the box.  The two copper plates do not touch each other.  Otherwise, you’d have one hell of a short circuit on your hands.

We’ve found this type of box to be somewhat problematic with certain cell types,  particularly cells with residual welding scrap on them from prior service.  We opt to  build our parallel groups first.  This includes measuring, cutting, and welding nickel strips, and completing all the parallel groups.

Equalize Overnight: Once we have all our parallel groups, we alligator-clip the positive of all groups together.  We then alligator-clip all our negative ends together.  Don’t connect positive to negative, or you will have a short.

Parallel groups equalizing overnight.

We then leave these parallel groups in a secure location so that they can all balance out overnight. If you have animals or children in the house, it is imperative you store your balancing batteries in a secure box or area.  We place ours in a large plastic tool box, all flat and not touching each other in the bottom of the box.  If we had children around them, we’d go even further in securing the project.  Remember that if these parallel groups get moved around and positive comes into contact with negative, you will have a short, and lithium batteries in a shorted condition will create a very large fire that is extremely difficult to extinguish.

Welding nickel strip onto parallel group.

Weld the Series Connections:  After a night of balancing, we test voltages to confirm equalization, and we check one more time for loose welds.  We then hot-glue one group to the next, paying attention to positive-to-negative flow routes.  Remember, you are now building the series part of the circuit, so mentally trace a route through your groups.  One positive to the next negative, and so forth, until you complete your series connections.

Measure and cut nickel strips

Series connections handle a greater amount of current than an equivalent parallel connection.  We use wider nickel strips for our series connections as a standard practice.  It’s beyond our current discussion, but you should always calculate your amperage requirements and size your nickel strips accordingly.  When we get into larger amp packs, we always stack our nickel strips to accommodate the more powerful current. In some cases, we use copper rods purchased from our local industrial metal supplier:  Alro Metal.

Check voltages again, confirming that your per-cell charge multiplies out to what it should be based on your series connections.  If all good, proceed with positioning the BMS unit (we use green kraft-paper tape as an insulator), and finish wiring up as detailed in your BMS instructions.

Helping Hands:  If you’ve spent more than a few minutes in this hobby, you already know that humans need at least one more arm and hand.  We use a 6-arm device when we’ve got a lot going on and need a whole lot of help.

Extra hands.

The nice thing about this particular device is that the arms are detachable.  We’ve found having one arm connected to our multi-axis vice to be a particularly useful piece of gear in our shop.

Ever need a third hand?

Hope you’ve enjoyed this short piece covering safety equipment, tools, and basic pack-building tips.  If you are a member of our group, and have an interest in purchasing some of the more difficult-to-find tools listed here, let us know.  We occasionally put together a purchase order for multiple units of something at wholesale prices, and we don’t mind passing those along to our members.