Running a DC-DC based computer off of a battery

Kmpkt

Innovation through Miniaturization
Original poster
KMPKT
Feb 1, 2016
3,382
5,936
Hey Guys,

Quick question regarding batteries + DC-DC power supplies. I've done a bit of reading and I'm trying to figure out how viable it would be to power something like an HDPlex 250 off of a drone or RC battery pack. I figure I'd need to use a 5S battery pack with a nominal voltage of 18.5V like this:

https://hobbyking.com/en_us/zippy-flightmax-8000mah-5s1p-30c.html

What I am not certain of is the following:
- Safety? I really don't want to blow a LiPO pack
- Discharge rate. If I anticipate needing a peak of about 360W for a gaming PC, then that means I need a peak discharge rate of around 30A correct?
- Charging - Any safe way to charge with something like a 19V charging brick or would I need a specific battery charger?

Cheers.
 

Rusty McFot

Trash Compacter
Jan 4, 2017
48
79
If you're planning to use a LiPo battery, there are some critical safety aspects you need to remember.

Charging: You need a dedicated LiPo battery charger which is designed for LiPo batteries. It will have built in protection against over current charging. Lithium batteries have a tendency to explode if charged incorrectly.

As lithium batteries don't have a linear discharge curve like NiCd or NiMh, you need to be very careful about how you far you let it discharge. If you have a DC-DC converter plugged straight into the battery, its likely you'll have no discharge monitoring. Unless the battery itself has built in under-voltage protection. Many LiPo batteries will have a small PCB under their heatshrink wrapping that contains its own protection circuit.

This can lead to dangerous situations where as the battery voltage suddenly drops (remember it have a non linear discharge characteristic), you get a very rapid increase in current. This rapid increase in current causes a lot of heat inside the battery, and can damage the batteries. You want the battery to be automatically disconnected from the circuit before this happens.

If you are worried about the high current, you can use multiple batteries in parallel to divide the current. 15A across two instead of 30A across one, or 10A across three, ect. Just remember you need the cell packs separated by a circuit of diodes that can handle the current, in case you have batteries at different states of charge or with older batteries not discharging uniformly with respect to the others.

I would be powering by at least 3 of those batteries.

I used to build field deployable 'systems' for various government departments, which used this principle to get longevity out of their equipment and make it all hot swappable, so there was no interruption when you disconnect a battery to replace it with a fresh one.

I also over engineer everything I design, but I can point you in the right direction anyway.
 

Phuncz

Lord of the Boards
SFFn Staff
May 9, 2015
5,863
4,920
You could also start with LiFePo4 cells, these are much more stable in the sense they don't burn down houses or people.
 

Kmpkt

Innovation through Miniaturization
Original poster
KMPKT
Feb 1, 2016
3,382
5,936
So looking at LiFePO4 cells, they look to be much heavier and more expensive than their LiPO counterparts. I'd prefer to go with the lighter alternative and obviously cheaper isn't a bad thing. How much more dangerous would LiPO be than LiFE. I know you can get PCBs to provide overvolt/overcharge/overdischarge protection on batteries. From what I've seen, LiPO will go up in smoke in these situations. Are there other risks using LiPO? Also what is typically used in Laptops/Cellphones/Etc?
 

Rusty McFot

Trash Compacter
Jan 4, 2017
48
79
Laptops and phones generally use Li-Ion.

They still have similar dangers if not wired up with dedicated protection circuits.

The dangers of LiPo and lithium batteries in general, are also health related. Lithium is incredibly toxic if you are exposed it in the form of gaseous venting. It's a known carcinogen and quite harmfull. Overheated or damaged batteries can leech lithium vapour.
 
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Phuncz

Lord of the Boards
SFFn Staff
May 9, 2015
5,863
4,920
So looking at LiFePO4 cells, they look to be much heavier and more expensive than their LiPO counterparts. I'd prefer to go with the lighter alternative and obviously cheaper isn't a bad thing. How much more dangerous would LiPO be than LiFE. I know you can get PCBs to provide overvolt/overcharge/overdischarge protection on batteries. From what I've seen, LiPO will go up in smoke in these situations. Are there other risks using LiPO? Also what is typically used in Laptops/Cellphones/Etc?
When LiPo's become unstable, they usually seem to grow a few sizes in dimension and vent gases. Often times, those gases get ignited and make it into a mini flame thrower.
 

SPK

Efficiency Noob
Mar 2, 2017
7
0
Hey Guys,

Quick question regarding batteries + DC-DC power supplies. I've done a bit of reading and I'm trying to figure out how viable it would be to power something like an HDPlex 250 off of a drone or RC battery pack. I figure I'd need to use a 5S battery pack with a nominal voltage of 18.5V like this:

https://hobbyking.com/en_us/zippy-flightmax-8000mah-5s1p-30c.html

What I am not certain of is the following:
- Safety? I really don't want to blow a LiPO pack
- Discharge rate. If I anticipate needing a peak of about 360W for a gaming PC, then that means I need a peak discharge rate of around 30A correct?
- Charging - Any safe way to charge with something like a 19V charging brick or would I need a specific battery charger?

Cheers.
Please tell as to why did you choose a nominal voltage of 18.5V, considering the maximum voltage used by PC components is 12V.
 

EdZ

Virtual Realist
May 11, 2015
1,578
2,107
19v is the 'standard' voltage used by laptop chargers, the main source of consumer packaged AC-DC bricks.
 

SPK

Efficiency Noob
Mar 2, 2017
7
0
19v is the 'standard' voltage used by laptop chargers, the main source of consumer packaged AC-DC bricks.
Thank you for pointing towards that. Please tell a suggestive 'nominal voltage' of a battery pack if used with a custom 12V AC DC Brick.
 

Kmpkt

Innovation through Miniaturization
Original poster
KMPKT
Feb 1, 2016
3,382
5,936
Most DC-DC power supplies should have a specced input range and it will be your job to figure out the battery to pair with it. The HDPlex units take 16-24V which is quite wide and makes them ideal for this sort of application. If you can point us to the unit you're using, perhaps we can help you more.
 

Dyson Poindexter

If there's empty space, it's too big!
Jun 25, 2015
55
62
Mini-Box has a similar product that might be useful for design inspiration: http://www.mini-box.com/OpenUPS2

Keep in mind that you must, must, must use a proper battery management system. Or at the very least, a normal hobby charger and LVCO on the discharge circuit.

I've ran a Pico-PSU off a 3S lipo before, and it does work just fine. I was monitoring pack voltage manually though.