Oooh ! Oooh! How do you know?
Thank you for asking! 
I usually conscientiously replace my car battery when it seems to be getting weaker and it’s of an age when that’s consistent with being worn out. In this particular case the battery was almost new, but a partially failing alternator was not keeping it properly charged and it suddenly failed completely after the car was sitting for a few days. There was not even a faint glow from the interior lights. My faithful Li-ion booster pack which had never let me down before did absolutely nothing.
I find it much harder with modern cars to tell when the battery is getting weaker since they hardly crank at all before the engine starts, so I can’t tell if the starter is slowing down. Add to that the fact that here in the desert batteries tend to die suddenly and early (used to be 2 years typically, but I think it’s better now that batteries in the passenger compartment is common) it’s hard to be proactive with replacement.
I personally put in the largest battery that will fit in the vehicle. If you put in the battery that came with the vehicle, it will last maybe 5 years. Put in an oversized battery and it will probably last the life of the vehicle (unless you’re like me and you have several cars that are more than 50 years old).
This of course does assume that your vehicle has enough space in it for a larger battery, which is not true of all vehicles. Some vehicles have very little room for the battery, and you physically can’t fit anything larger than the el-cheapo little piece of crap battery that they put into it when it was built.
They intentionally under-size car batteries these days to save weight and to save space under the hood. People just expect batteries to last 3 to 5 years and that’s it, and the car dealers are more than happy to sell you a new battery to replace it. I don’t recommend pro-actively replacing your battery with a larger one, but when you get to the point of needing to replace it, spend a few extra bucks so that you never have to replace it again after that.
The battery backups they sell as “Whole house generators” are the ones that annoy me. Jackery is a popular brand. They advertise their Explorer 2000 Plus as “The Only Home Backup Power Solution You Will Ever Need”. Oh, come on. It’s a nice battery/solar unit. It puts out a max of 3000 watts. It has 4000 watt-hours capacity so several hours of use. It takes 6 hours to recharge on a fully sunny day. That certainly does not sound like the only home backup solution you will ever need. AND IT COSTS $4500 !! Every time my wife sees that commercial she says “OMG we need one”. I remind her of the mighty Onan 6500 watt generator I have and how much the Jackery costs and how long it lasts.
For that much you can get a real automatic whole house generator like a Generac. For $600 you can get a decent sized portable generator. Either will give you full power for as long as you have fuel, day or night. For $200 you can get a deep discharge battery and a power inverter that will do everything the Jackery will do. A solar panel to charge it is another $150.
I try to do that but unfortunately the current vehicle has limited space for the battery so my options are limited. I do buy the highest quality, highest capacity battery I can get, I typically get about 7 years out of them.
I think I fixed my jumpbox!
I knew it was dead because I charged it overnight and it went into maintenance mode. When not getting the household power, the built in light was dim and battery condition was red. As a further test I plugged it in and sure enough the built-in light was super-bright.
I forgot about it and left on a 1 week vacation. Working in my garage, I noticed it was still plugged in. Ahh well, I should unplug it and dispose of it in a legal way. But just to see, I turned on the light. Bright as if plugged in. And the condition light is fully green. I’m going to leave it unplugged for a week and see what happens but maybe it just needed a 7 day charge.
Assuming the internal battery is sealed lead-acid, I’d bet that you now have a jump box of near-normal cranking capacity but with a pretty quick standby discharge rate. IOW right after being fully charged it totally will start your car. But you’ll need to recharge it every 2-4 weeks or else it’ll be too dead to do anything when you next need it.
I’ll also note there’s a vast, vast difference between a battery trying to crank a car engine and that same battery trying to light an LED flashlight. The LED test proves the battery can produce decent voltage at tiny amperage. There’s not a good way at home to test a battery for high amperage cranking capability.
And now my turn with a defective jump box.
Awhile ago (2-3 years) I bought one of these Lithium-ion jump boxes that rides around in my trunk:
Three months ago I pulled it out and discovered it was nearly dead. So I charged it up and created a reminder task to recharge it every 3 months. Since this was my first go-around with periodic recharging, the “3 months” was just a guess, and my real intent was to see how fully charged it was or wasn’t after 3 months then adjust the interval going forward as needed.
Anyhow, three months from then is now now. So today I pull it out out, find it stone dead, and notice the telltale signs of a bulging battery: the case is all misshapen to the point the membrane-style buttons don’t push. It’s trash, and semi-hazardous trash at that.
So now I need to decide if I care enough to buy another one. I bought it the first time mostly as an impulse purchase while I was in a battery store for something else altogether.
Just want to chime in to say vehicle battery testers are really cheap now. Mine has paid for itself many times over.
My only compliant with them is that they’re not temperature-compensated; they assume the battery is at room temperature. If the battery is not at room temperature, the reading will be off a little, but it’s still a lot better than nothing.
I’ve got a similar lithium-ion jump starter, though a different brand, and had the issue of “How do I keep this charged, but still carry it around in the car?” Mine uses a USB C connector to charge, so I connected it to my 3-port cigarette lighter USB power unit below the dashboard. The socket is only on when the car is running and this keeps it topped up very well. It’s always ready to go. The concessions I made to safety are (1) I keep the unit in sight on the floor in front of the passenger seat and (2) I have a short length of wire tied to it so I can quickly drag it out of the car if it develops a problem.
My wife was a little leery about the arrangement, but I pointed out that it doesn’t charge at all when the car is off. And would we rather have it catch on fire (if it is so inclined) in the car while we’re there or at home when we’re out of the house or asleep?
Also, even if you’ve bought a new car you should check the battery’s age. I discovered my “new” vehicle had an older battery than I realized. My truck was built in late 2021, languished on the dealer’s lot all of 2022, and was purchased by me in early 2023. The battery was dated 10/21, almost 18 months old when I brought it home.
I watched it carefully due to this, and a few months ago I started seeing voltages below 12.0 in the mornings. So I replaced it rather than wait for failure.
Agreed. I bought the largest AGM that would fit in the space. 850 CCA, 115 Ah, and 140 minutes reserve. I still carry a lithium jumpbox though, and it’s tested every 3 months or so. My riding mower eats batteries for some reason, so it’s always ready to volunteer as a test platform.
Hmmm.
AIUI Li-ions can choose to catch fire at any time. Perhaps more likely while charging, but not that much more likely. There’s no good answer for where to store it if it can burn at any time.
I admire your emergency preparedness dedication by attaching a jettison wire and keeping it underfoot and in sight while driving. That is far more than I could muster. The habits of one’s career do carry over into our non-work lives, don’t they? 
OTOH, I do question the wisdom of leaving it plugged in all the time while driving. The battery charging controllers in e.g. phones and EVs are very sophisticated and can meter charging such that being perpetually plugged in while fully charged is not a problem. The charge controllers in lesser machines are … lesser. Often much lesser.
I have an e-bike with a big battery and a big boxy charger. With explicit warnings to NOT leave the charger plugged in after the battery is showing full. Why would that be? They cheap-Charlied the controller. If they did that on a >$1K item, how fancy is the charge controller on a $50 or $100 jump box? “Not very” would be my opening guess.
I absolutely agree, LSLGuy. But the USB charger is completely off when the car’s ignition is off. There is no voltage at all to the jump starter unless the car is running. It may as well be unplugged completely.
I would never leave something like that charging in my house while I’m in the yard or away, just like I don’t leave the oven or the dryer running such circumstances.
Yes, the jettison wire is a technique I’ve used a couple times. I’ve done the same thing in similar circumstances.
I didn’t quite make my point clearly enough. Sorry.
IMO … Leaving the jump box’s USB charger plugged in and powered only when you’re actively driving, but also always when you’re actively driving is still charging it far more than it needs. At least it will be if this is a car you drive regularly.
Assuming the jump box has a shitty cheap charge controller you’re still overcharging that battery every time you drive. Which will reduce the life of the battery and perhaps provoke a swelling or fire unnecessarily.
I’m much more comfortable with leaving the jump box in the trunk & bringing it into the house to top up the charge every couple of months.
YMMV of course; I’m not sure there’s an objective answer to which is the safer procedure overall. I’m just trying to communicate a position clearly, not necessarily trying to persuade anyone of its correctness.
NOCO is a good brand in general, though. I use their Genius series 3-bank charger onboard my golf cart. Made in Cleveland.
Good to know. Thank you. I had wondered just that.
I appreciate the concern, LSLGuy, but I’m semi-retired and drive very little. No commutes for me anymore! Plus, the manufacturer states in their instructions that it’s fine to leave it plugged in all the time. It has a pretty sophisticated charging circuit and digital display for status.
But I wholehearted agree that very few items should be left charging all the time unless they have “smart” charging features. In my case, I probably won’t have it charging more than 30 minutes a week.