One of the problems we face at Hackerfarm is the need to constantly water seedlings …
Here at Hackerfarm, we use 18650 rechargeable lithium ion batteries for just about everything. We also hate throwing out rechargeable batteries if we can avoid it. Recently we were going through our lithium-ion battery stock and found out that a bunch of them were completely discharged and could not be revived. The battery chargers we connected them to were not charging at all. For lithium-ion batteries, the fully charged cell is around 4.2V and a discharged cell is around 3.0V. A deeply discharged cell has fallen below 2.5V and most lithium-ion battery chargers can’t revive cells that have discharged this deeply.
Once a battery reaches below 2.5V, the low voltage cutoff threshold, it goes into a sleep mode where the protection circuit shuts down all operations. Most battery chargers won’t charge a battery below 2.5V which means the battery you have is essentially useless. What you have to do is provide enough charge to the battery to boost the voltage above the low voltage cutoff threshold. Once this happens, the protection circuit kicks in and the battery can be charged normally. This is also a great way to salvage 18650’s, especially from dead laptop computer battery packs.
Some chargers have a “boost” feature to charge a deeply discharged battery and revive it but there’s actually an easy way to do it. You just connect the battery to a voltage source (3.5~5.0V) and charge it for around 5 minutes to get the voltage up. Once that happens, you can put the battery inside a charger and get it to charge normally. One caveat is that you should be present when you do this the whole time. You don’t want to leave the battery connected to an unsupervised 5V source since it could overcharge and cause a fire. Also, you’ll want to supervise the charging of the batteries inside the charger to make sure they are okay and don’t heat up or inflate (outgas).
Here you can see me reviving some dead 18650 batteries that have been in storage for too long. This particular battery is at 0.423V which is way below the 2.5V cutoff. It cannot be charged at all when placed in the charger.
I’ve set my DC power supply to 5V in preparation to connect to the lithium ion battery. This will be the target charging voltage.
I connect the 18650 battery to the DC power supply via alligator clips. I’m using a single 18650 lithium ion battery holder to hold the battery as it gets charged. You can also go ghetto and solder wire leads to it or my favorite is to use neodymium magnets on the positive and negative electrodes. Then just drop the alligator clips on the magnets. It’s also important to have a timer handy and make sure you don’t leave the battery charging unsupervised. Overcharging will kill the battery and potentially cause a fire.
Once the battery is connected, you can see that the power supply voltage drops to 3.7V. I’ve current limited the supply to 1A so that it doesn’t quick-charge the battery too fast. This can also create outgassing. You can also use a 5V, 1A DC wall wart to do the same thing without needing a fancy, schmancy power supply but it is much nicer.
After the 5 minutes are up, you can see that the batteries have now reached 3.367V. The batteries charge quickly up to this point because there’s not a lot of energy storage capacity between 0.5V and 3.0V. You can probably get there in around a minute of charging actually. Once you’re above 2.5V, its okay to slap it in a conventional battery charger.
Now I can just throw them in my rapid charger and charge it up completely.
No waste and you can salvage other people’s batteries that they think are dead. Yay!
Next stop…DIY Powerwall…