The weight and size difference is very, very different between these two, but to a person with only surface-level knowledge like me, they have nearly the same specs.
Context: I have a UPS (Tripplite INTERNET350U) that has a very dead battery. I’ve been playing with the idea of a replacement but I can’t for life of me justify spending $30 on a brand new lead acid battery for such a low-performing UPS with no way for me to perform maintenance charges on it. But this type of battery is so niche that I can’t find any used or refurbed ones.
I KNOW that Li Ion is way more dangerous than lead acid when it comes to spontaneous combustion and heat. So I will probably not go thru with this. But…. That doesn’t mean I don’t wanna try it just to see if it actually works. But even still, I feel like something is telling me that even if I match the aforementioned specs, it still won’t work as such?
Comments
The charging circuit is totally different. Also, Li-ion need a “cell balancer” if you have more than one cell in series, or the life will be short.
lithium has a much lower internal resistance which means more current will flow when charging from the internal lead acid charger that the unit was not designed to deliver or sustain
the voltages will be wrong also which will also limit charge
In terms of output? Not much if the li-on are connected in series.
You will have to check the electronics and see how low the voltage can go because 3.7 x 4 is still less than 12. But the lead acid will sag too as it discharges.
For something like this you’d probably want LFP cells since sitting at full charge for extended periods will hurt them less. The voltage drop as LFP discharges is also quite minimal till it gets very low.
Also, one key advantage to lead acid (why they’re still widely used in cars) is their ability to dump huge amounts of current all at once which is needed to overcome the high current draws of a starting motor. Not sure if this is relevant for your application though. But if you’re planning to use cylindrical cells make sure the power output of a given cell is sufficient for your application. If it’s not, you may have to double up your cells and then hook those pairs in series.
Finally, you will need a BMS to properly manage the lithium cells.
If your UPS is so old that you can’t find batteries for it, it’s surge protection circuits probably are worn and no longer provide protection, so it’s probably best to replace the whole unit.
A charger meant to work with lead acid batteries is not going to be able to handle Lithium Ion. Best case scenario is that it would refuse to charge. Worst case scenario is explosive fire.
A device that recharges batteries need to be designed with the battery type in mind to recharge them in a safe way. Safe in this case means that the batteries should last a long time and not have a catastrophic failure, then you recharge the batteries.
Even if they are not charged, how they behave when the voltage drops and the discharge is different. The device could use them so the voltage drops to low or it might turn off before all usable energy is extracted because it detects an empty battery as if it were a Lead-acid battery
So, do not put just 4 Li-ion batteries in series in a device designed for lead-acid batteries. The worst-case scenario is a fire.
If you want to use Li-ion batteries, what should work is Li-ion replacement batteries 12-V led-acid batteries are used in cars, morocycles etc. They include batteries and a charge controller to handle the charging and discharging. Even then, look at the battery specification in regard to what voltage it can handle, and you need to determine what voltage the UPS might put out. They are quite expensive, and I would guess not something you want to use.
A more reasonable upgrade is to use a larger Lead-acid battery. That should work fine, a larger total capacity just means the charge time and time to deplete it is longer, the voltage curve looks the same for both, the time is just different.
All lead-acid batteries are not the same there, the most common are starter batteries for cars, which can handle a high current. There are also Deep-cycle batteries optimised for lower current over longer times. They can alos be discharged to a quite low voltage. They are often called leisure batteries because they are used as a power source in for example a camper. There is alos startr batteries optimised for high current over a short time.
You will find lot os people online modifying a UPS with larger external lead-acid batteries