Replace the lead gel battery with the lipo battery

[sfiga69]

Hello guys,
I have 20 LIPO 6.75Wh batteries.
I am wondering if it was possible to replace the battery of my alarm system with a parallel and series combination of these LIPO batteries.
The original battery is a 6V 3.5Ah lead gel.
I thought a series of 2 packs of 4 batteries in parallel for 27Wh and 7.2V with a protection circuit like this:
https://www.aliexpress.com/item/32996386019.html?spm=a2g0o.cart.0.0.7b8e3c00ue6p4o&mp=1

Do you think it's possible?
are there any contraindications?
thanks for your advice

[CocaCola]

Sure it's possible, but you need to change the alarm system charging circuit to an appropriate LIPO charging system and best to also implement a LIPO protection circuit...

I see little benefit to the conversion, over just swapping out the lead-acid and calling it a day...

[Sideshow Bob]

I agree with Cocacola. It can be done, but it will be in the same category as putting wings on a car in order to make an airplane. It is also a safety aspect to this. One thing is making something on your workbench and having fun while doing it. But this would be something that would be left unattended in working condition for a long time. In such situation you shall be very sure about what you are doing.

[sfiga69]

Can a protection circuit like the one I indicated could be enough?

[CocaCola]

Can't say if that module will work in theory probably in practice maybe, as Sideshow Bob said used in an alarm system this will be unattended 24/7 for years, lithium batteries if not properly maintained/charged become a huge fire risk...  From experience, many of these Chinese modules are the bare minimum design...  You can pull the datasheets for the parts on the module and get a feel for the design and if it's done decently...  But, also consider the page does not appear to give any real info, basic info like input charging voltage min/max isn't provided, plus will the charging circuit on your existing alarm system put out enough amps and volts to properly run this module...  Also, consider many times they use proprietary chips that you might not be able to get datasheets on to even verify the circuit...  Does your existing alarm system have some kind of battery charging failure circuit that might not be compatible with a LIPO mod without further mods to main security system board itself so it doesn't fault out?  I'm not saying it won't work, I simply don't have enough info to give an educated opinion...

This type of project is one I won't bother with, I would swap out the lead-acid and be done and on the next project already...

[papy_bidouille]

why wanting to replace something that works well, which has all the security possible, is inexpensive and always accessible, easy to find by a product difficult to implement without making a serious study of the possibilities? cordially

[Sideshow Bob]

@sfiga69 Your alarm system will have battery supervisory circuitry and charger circuitry designed around a 6V 3.5Ah lead gel. If properly designed it will ensure safety and longevity for that kind of battery ONLY. Each battery chemistry will have its own specific approach to how this is done. Charging circtury may use constant current, constant voltage approach. But we will also quite often see a combination the latter. I.e using constant current up to some charge level, and then switching of to constant voltage for maintenance charging. The voltage and current level depend things like battery chemistry, capacity and so on. So hooking up your alarm system to a completely different battery chemistry will for certian cause all kind of problems. Using a BMS/charger circuit is mandatory for LIPO batteries. But a using a poor designed or wrong specified BMS may give false sense of safety. Like if the short circuit protection current level is to high so it will never be tripped. This may very well cause thermal runaway(use google to find out more). So bottom line the BMS you plan to use are way to cheap even accounting for mass procution to be trusted IMO. So as we all have said over and over again. Do not do it. In worst case you may end up burning down a house/building. And if insurance company finds out it is your own fault. You will be in trouble
Edit: Just to say that. Properly designed comercial lithium ion batteries used the correct way. I will regard as safe as the industry have learned to minimize the fire and explosion risk. 

[pickit2]

working on high security we used to replace lead-acid batteries every two years. there was always plenty of use in the batteries we took out.

[fpgaguy]

I've created lithium battery fires with LiPo's ... It's not pretty. Nasty HF, other phosphorus/flouride containing compounds and carbon monoxide are present and of course a large amount of heat.   Stick with what the circuit is designed for, and has many hours of testing for safety and handling of faults. Unless you are planning to do all this testing and design I would not do that.
   There is a reason I have to mail large LiPo's uncharged or by ship/truck.

[sfiga69]

I understood. In my total inexperience with the lipo, I believed that, given the wide spread, they were more manageable.
At this point I will use them to replace my drone's batteries.
In the case of problems I will see fireworks   but the house is safe.
thanks to everyone for the precious advices

[Sideshow Bob]

I understood. In my total inexperience with the lipo, I believed that, given the wide spread, they were more manageable.

Nope on the contrary

These cells behave as grumpy primadonnas regarding charging and discharging. And if not threated correct they just go off (literally) or just refuse to work any more. There are more forgiving batteries.

[token0]

LiFePO4 is much safer than lithium polymer. Though it has different voltage (3.6 V fully charged, ~3.0 V mid, ~2.0 V "fully discharged") and would have different charging circuit. But for alarm I'd stick with lead acid.