Cheap booster pack

[Connecteur]

I picked up a cheap booster pack in a thrift store for a couple of bucks.  It seems to take a charge and hold it, but it came with no booster cables.  I got a cheap set off Amazon, and it seems to deliver good current. It burned up a paper clip in a second.  It seems to have no booster switch of any kind or diodes to prevent back current. It always live.

I'm thinking I could attach it to my car battery to add capacity. Bad idea?

[TimFox]

Probably.
In use, it will discharge along with the main battery it parallels.
Better kept in reserve against surprises.

[Connecteur]

The idea is just to go longer before parasitic drain takes it too low to start.  I can't do anything about the parasitic drain; it does live updates, and if I'm in a poor reception area, it drains the battery fast.

[TimFox]

What is the capacity of the booster compared with the capacity of the main battery?

[Connecteur]

What is the capacity of the booster compared with the capacity of the main battery?

No idea. It looks like about the mass of 4 18650s.
I can't find a thing about it online.

[Stray Electron]

   There is at least one video on U-tube of someone using a couple of 18650s and drawing 500 amps from them with a dummy load to replicate jumping a dead 12 V auto battery.  The 18650s don't have a huge amp hour capacity (compared to an auto battery) but they WILL deliver 500 Amps for something like 30 seconds which is more than enough to start a car (unless it has other issues). I strongly suspect that the batteries will be ruined but it's still cheaper than calling out a tow truck or a mobile mechanic. 

  I would just make sure that you use some what longish leads and keep the battery pack away from your car if you attempt this because it will get extremely hot and melt any nearby plastics and very possibly start a fire.   

[Stray Electron]

Probably.
In use, it will discharge along with the main battery it parallels.
Better kept in reserve against surprises.

  X2.  Lithium batteries don't like heat and can do BAD things when they're overheated. Don't install them in the engine compartment and try to keep them somewhere where they will stay cool. 

[nvmR]

Although LION is better than LIPO in this use case. In terms of heat. LIPOs puff, while LION doesn't. I would validate what is inside, 18650s or LIPO.
If you have access to an outlet, maybe a trickle charger is the solution, to charge the battery from it while the car is off. Not sure how the alternator will like this though.

[Connecteur]

   There is at least one video on U-tube of someone using a couple of 18650s and drawing 500 amps from them with a dummy load to replicate jumping a dead 12 V auto battery.  The 18650s don't have a huge amp hour capacity (compared to an auto battery) but they WILL deliver 500 Amps for something like 30 seconds which is more than enough to start a car (unless it has other issues). I strongly suspect that the batteries will be ruined but it's still cheaper than calling out a tow truck or a mobile mechanic. 

  I would just make sure that you use some what longish leads and keep the battery pack away from your car if you attempt this because it will get extremely hot and melt any nearby plastics and very possibly start a fire.

I wouldn't fret. My car doesn't draw that many amps.

[NE666]

If you have access to an outlet, maybe a trickle charger is the solution, to charge the battery from it while the car is off. Not sure how the alternator will like this though.

Not only would the alternator be just fine, it's the really the correct solution here.

This type of situation, besides natural self-discharge during prolonged periods of off-road storage, is the reason that 'onboard/maintenance chargers' exist.  Of course, as has been pointed out, the crux is whether or not you have a mains outlet in proximity to the vehicle.

e.g. https://www.amazon.co.uk/dp/B08FCV1Q2Y perhaps in conjunction with https://www.amazon.co.uk/dp/B07TMR5PZ6 for convenience.

[Connecteur]

If you have access to an outlet, maybe a trickle charger is the solution, to charge the battery from it while the car is off. Not sure how the alternator will like this though.

Trickle chargers have destroyed my batteries before. I prefer to use a battery maintainer, which measures the state of charge periodically, and delivers a top-up charge as needed.
This booster pack is for the times when my car is not near an outlet.

[bdunham7]

I'm thinking I could attach it to my car battery to add capacity. Bad idea?

What model car and what type of battery does it have?  A booster like that will only result in a very small increase in the overall capacity so it will be more effective to keep it separate and available if the main battery becomes depleted.

[Connecteur]

I'm thinking I could attach it to my car battery to add capacity. Bad idea?

What model car and what type of battery does it have?  A booster like that will only result in a very small increase in the overall capacity so it will be more effective to keep it separate and available if the main battery becomes depleted.

It's a flooded cell lead-acid battery with 33 Ah capacity. The booster pack has a specified capacity of 12 Ah, which is significant to me.

I have a separate booster pack for the purpose of starting the car as needed, but the purpose of this apparatus is to lessen the trouble of hooking that up.

[NE666]

It's a flooded cell lead-acid battery with 33 Ah capacity.

Tbh, I think that's your problem. 33Ah is nothing for a car battery - even the small Type 12 on my Smart Roadster is rated at 50Ah. The Type 096 fitted to my Volvo V8 is 77Ah.

I'd be looking toward replacing it with the highest capacity main battery which physically fitted the vehicle, rather than applying a sticking plaster which may or may not hold up over time.

[bdunham7]

It's a flooded cell lead-acid battery with 33 Ah capacity. The booster pack has a specified capacity of 12 Ah, which is significant to me.

I have a separate booster pack for the purpose of starting the car as needed, but the purpose of this apparatus is to lessen the trouble of hooking that up.

You previously said:

No idea. It looks like about the mass of 4 18650s.
I can't find a thing about it online.

I assumed that meant it probably had a 3-5AH capacity at most.  But from your numbers it would appear to add 30% capacity or so if those numbers aren't fake.  If that's helpful, then go for it.  However, you probably  can't leave it connected all the time as I doubt it is directly compatible with the car's charging system unless it has some heavy-duty BMS.  How do you charge it and what is the open-circuit voltage when it is fully charged?