So here is what I'm trying to make I need a portable UPS for my portable DVD player 12VDC in 12VDC out. The reason for this is my car's 12VDC outlet looses power whenever the ignition is switched off, even switching it to accessory will kill the power. This plays havoc with the portable DVD player. Try telling a five year old that her show died because you needed gas. Rather than rewiring the outlet direct to the car battery what I want to do is build a small UPS out of some Lithium ion 18650 cells that I have left over. Now I'm planning to connect them with one of these:


Which should protect them for charging. Then use a dc-dc up converter to get me the 12VDC output I need and put a voltage regulator on the input to ensure I've got a stable ~12VDC on the input to supply the charge. That should I believe give me up to 4A to draw for my DVD player which should be more than enough for it, the excess is then available to keep the batteries topped up.

I just want to make sure I'm not missing something that will start say a bad lithium fire. I've done something similar with some sealed lead acid batteries but they are far more forgiving than Lithium ions. However, they are big and heavy I want to keep this small and light for travel it's not like it has to run the DVD player for hours just keep it running when we need to make pit stops. Because trying to find the exact instant that the show stopped because children only care about that sort of thing when it's their shows.....

I would love some feedback to let me know if there are some considerations I have missed.



2 Answers 2


This advice is applicable for those who do not want the design challenges or risks of working with lithium batteries or sourcing reliable protection circuits. There are simpler ways to go about this because a lead-acid charging system is already present; i.e. the alternator output is just right to charge a lead-acid and close enough to charge NiCd/NiMH.

Auxiliary battery (which can be portable)

Choose a battery which leverages the fact that your charging system is just right for a lead-acid battery. The obvious choice is lead-acid, and they come quite small. However, as it works out, ten NiCd or NiMH also will charge quite effectively off a system intended for lead-acid charging.

Of course, this should not be permitted to drain your main battery when the car is off. Nor allow the little battery to "help start the engine" as that would cause excessive overcurrent. Fortunately the annoying misfeature which cuts off your child's movies is well suited to this: hot-in-run circuits cut power in "off" and "engine start". Watch out if moving this to a car that doesn't have that -ummm-feature.

The only other issue is charging overcurrent if the battery is depleted, but that will vary by battery. A well chosen resistor will take care of this.

You could add a Schottky diode if you were particularly worried about reverse current.

Now, let's talk chemistries.

6-cell lead-acids are readily available in packages under a pound. The upside is it's a perfect electrical match and will perform well, and are cheap! The downside, besides their notorious weight-to-capacity, is they really do not like to be bottomed compared to other types. Unlike Lithiums where 2000 25% cycles and 1000 50% cycles are equivalent, lead-acids are much more aged the deeper the cycle goes. I am already talking about deep-cycle types. Don't believe the salesmen (unless they'll give you a warranty), talk to the solar, narrowboat, vanlife people: "size for 25% drain, 50% rarely" is common knowledge. Refs: a b c d e They are also damaged by being left in a discharged state - and can freeze if discharged.

It's a happy accident that 12V lead-acid charging systems are "close enough" for a 10-cell nickel-cadmium or NiMH pack. They will not overcharge and will slightly undercharge, but that doesn't present a problem. I use common 12V solar charge controllers to charge a large NiCd wet cell that is almost 40 years old, works like a champ. And like most battery chemistries, NiCd/NiMH aren't hurt by bottoming, in fact factory advice is to store them discharged. (that'll happen anyway, they self-discharge over 3-6 months.) Both NiCd and NiMH can output a reasonable amount of current if asked. NiCd capacity is better than lead-acid but not fantastic; NiMH is better. (keep in mind you will not be finish charging these to absolute max). None need charge or discharge protection circuits.

Lastly, since portability is a goal, I'd want to fab this into a package that attaches to the DVD player. (and maybe houses a couple of DVDs). Packaging is half the fun! I'd rather try to site ten C-cells than an SLA, honestly.

Time-delay relay to an always-hot connection (NOT portable)

This assumes you can alter the vehicle. It is likely they provide grab points to get the necessary power, securing the new wires is up to you. In this case, you use the main battery, but add an accessory outlet (or alter the existing outlet) so it is fed by an "always-hot" circuit through the relay contacts.

The relay coil is tied to any "hot-in-run" circuit, energized when the engine is in "run" or accessory (not off or cranking). That closes the relay, and holds it; when power is removed the relay stays closed for X more minutes, then shuts off. You need to find a value of X that makes sense for you.

The purpose of this is to avoid running down the main battery from accessories left on all night, so you can afford to be fairly generous - 30 minutes would be safe for a DVD player.

  • \$\begingroup\$ Sounds as if you missed this part: "I want to do is build a small UPS out of some Lithium ion 18650 cells that I have left over." Not sure why you go on and on about lead acid. RE: "use any chemistry but lead-acid " You do realize there are different types of lead acid, cars use starting batteries, UPS Backup use stationary, EV use deep discharge and AGM lead acid is made for deep discharge (or bottoming as you say). Not that it matters because he is going to use Li-ion. \$\endgroup\$ Mar 22, 2017 at 0:59
  • \$\begingroup\$ sorry, I don't want to be combative, I'm not trying to compete with you and I apologize for doing so. You are right, I missed that, I am sure he will enjoy your solution greatly, and you deserve best answer. His problem is common but his constraint of must use Li-ions is uncommon... given SE's format, I expect others will come along who will not have that constraint, and I hope they find my answer helpful. The home-power community is at wit's end trying to find bottoming-tolerant batteries and your AGM info will be well received. Thanks! \$\endgroup\$ Mar 22, 2017 at 5:03
  • \$\begingroup\$ You are correct that lead acid is a good battery for a UPS. A UPS also is not going to allow the battery to be discharged 100%. The unit does have to make it through the warranty period. Secondary batteries all have a discharge cutoff voltage where discharge should stop. I believe my answer is the best way to solve the problem but I do not believe the OP will pay $75 for the load switcher demo board. I do not mind someone being combative as long as it does not get personal. \$\endgroup\$ Mar 22, 2017 at 15:52
  • \$\begingroup\$ I'm going to side with Misunderstood on this one. The information you provided is all valid and useful to be sure. In fact most production UPS devices use sealed lead acid. My issue with SLAs is weight/size always a problem when you want portable that's why most portable electronics use lithium ion or lithium polymer. So my choice to use Li-ion is to support "portable UPS" I want it to move not be part of the car. I know it's not the easy solution or even the standard one choosing Li-ion means more complex. People that want easy don't build these things they buy them. \$\endgroup\$ Mar 22, 2017 at 20:08
  • \$\begingroup\$ Sorry, I see now where my time-delay relay advice would be inapplicable. By my reasoning there are small 6-cell lead-acids which are under 500g (so physically small) and under $10, and ten AA NiCd's are also quite small, any of these are no bigger than four 18650's. I must admit I've not worked with 18650s, perhaps I am grossly overestimating their size. I wouldn't have imagined a problem packaging any of them in a "case" which also holds the DVD player, but that's just me thinking out loud again. \$\endgroup\$ Mar 22, 2017 at 20:36

The car battery is just closer to 14 volts.

Use 4 Li-ion batteries but do not charge them 100%. Keep the charge voltage just below car battery voltage.

What you are looking for is a 12V UPS.

Feed the 12 Volt outlet with the car battery and the Li-ion through a Positive High Voltage Ideal OR. When the car battery drops below the Li-ion, the outlet will be powered with the LI-ion. When the car battery voltage picks back up, it will switch back to car battery.

For the Diode OR look at Linear Tech LTC4412 Low Loss PowerPath™ Controller
Another candidate: LTC4419 PowerPath Prioritizer

Very similar device demo board: LTC4416 DUAL “IDEAL DIODE” POWERPATH DEMOBOARD
Another Demo Board: LTC 4419 Dual Input Power Path™ Prioritizer Demo Board

  • Automatic Switching Between DC Sources
  • Simplifies Load Sharing with Multiple Batteries
  • Reverse Battery Protection
  • High Current PowerPath Switch
  • Industrial and Automotive Applications
  • Uninterruptible Power Supplies

LTC4412 Low Loss PowerPath™ Controller

  • \$\begingroup\$ Thanks this looks like it will allow me to have it only drawing from the batteries when the car isn't providing me the power. The pinouts don't seem to line up with the datasheet for the LTC4412 but I can see where that's going anyway. I think I can make this work. \$\endgroup\$ Mar 22, 2017 at 19:58
  • \$\begingroup\$ @JamesPerry You could override the SENSE pin to select either source. Or just unplug the source you do not want to use. If you would want to use just the Li-ion batteries. I will fix the datasheets, links to wrong one. \$\endgroup\$ Mar 22, 2017 at 20:11
  • \$\begingroup\$ @JamesPerry The more I think about it, it's a good product, can you make me one too? My car will not power the cigarette lighter sockets with out the car on or accessory. Sometimes I want to leave my phone in the car to charge and I cannot do that. \$\endgroup\$ Mar 22, 2017 at 20:31
  • \$\begingroup\$ Disregard the pinout issue just realized the drawings were leading to the LTC4419 not the 4412. \$\endgroup\$ Mar 22, 2017 at 20:38
  • \$\begingroup\$ I will let you know how it works out and my final drawings for it. \$\endgroup\$ Mar 22, 2017 at 20:40

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