Battery for PC motherboard - Page 1
Forum section: electronics / general

author: eeguy
author_rank: Regular Contributor
timestamp: May 30, 2020, 04:14:02 pm
content: Hi, I want to use a Lipo battery to power a PC motherboard which takes 12 to 19 Volt DC with a maximum current of 10A. Lightweight is the most important criteria. Should I be using a 3S or 4S Lipo? What capacity and discharge rate should I be looking at if I want to run the PC for about 25 minutes? Given mAh*C of two batteries are similar, should I be choosing one with higher mAh or C value?

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 30, 2020, 07:03:06 pm
content: 4S will nicely fit into the voltage range specified. The C rating only needs to be 2 or higher for your application, certainly not very demanding. You'll need about 5Ah to meet your requirements, but batteries are cheap so it doesn't hurt to buy more and run them more conservatively so they last longer. Use a 16V current limited supply (4V/cell) to charge the batteries and power the load at the same time.Here's one pack that will work nicely:https://batteryhookup.com/products/pack-with-36-sanyo-1150mah-lithium-ionYou'll probably need to replace the BMS with a universal one unless you can hack the proprietary one that's included.

author: eeguy
author_rank: Regular Contributor
timestamp: May 30, 2020, 07:14:11 pm
content: Thanks. The link you provided has a 1.15Ah but you mentioned that I would need about 5Ah. I am a bit confused. Can you clarify please? What is the problem with lower Ah such as about 1.5Ah and C over 60?

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 30, 2020, 10:42:22 pm
content: 4S9P means 4 in series, 9 in parallel. Therefore, the total capacity is 10.35Ah.With a 10A load, theoretically 5Ah will run it for 30 minutes. In practice, it will be slightly lower due to losses.

author: mariush
author_rank: Super Contributor
timestamp: May 31, 2020, 06:58:34 am
content: Quote from: eeguy on May 30, 2020, 04:14:02 pmIt would be best to figure out how much power the components in your computer will consume. The motherboard itself won't consume 10A of current at 12v, that would be 120 watts, which is unlikely.Most likely that current rating is calculated using the highest power processor that can be installed onto the motherboard, and with two or more mechanical hard drives powered from the motherboard, all the memory slots populated and so on.A mechanical drive will use up to around 6-8 watts, so if the motherboard has power connectors for 2 sata ports, they may very well reserve 1-1.5A of that rating for the sata drives.If you're gonna use a single SSD, that will consume on average less than 0.5w, only when writing to the flash memory cells it will consume up to around 3w for the duration of writing to cells.A ram stick will consume 2-3 watts...Figure out how much power the processor consumes, if the processor is not soldered ... if you have a dc in barrel jack connector you could use a clamp meter to measure the actual current your board currently uses ... just separate the wires and place one wire in the clamp meter.Alternatively, get a power meter, they're cheap, can be bought for under 10-15$, and those will tell you how much power the power adapter consumes to power the computer.Examples (first results on amazn, not necessarily cheapest or the best but they'll do the job):https://www.amazon.com/Electricity-Monitor-Voltage-Overload-Protection/dp/B07DPJ3RGB/https://www.amazon.com/BALDR-Electricity-Monitor-Power-Energy/dp/B01JLT8NSG/https://www.amazon.com/Poniie-PN1500-Electricity-Electrical-Consumption/dp/B07VPTN8FZ/So use your computer for some time, then get that max current consumption. Take around 5-10% off because the power adapter won't be more than 95% efficient, and then divide that number by the output voltage and you'll get your current.So for example, let's say you have a 12v power adapter and the meter shows 25w max power consumption. Assuming 95% efficiency, that means only around (25 / 100) * 95 = 23.75w were actually used by motherboard ... that's a current of 23.75w / 12v = 1.97ABut for safety, you would not use a battery with 2A max, you would probably use one that can do 2.25-2.5A at the very least.If it's 19v, your current will be lower .. those 23.75w / 19v = only 1.25AKeep in mind also that battery voltage will go down as it discharges so your battery will also have to be capable of supplying current at its lowest voltage.For example, you may have a lithium battery that's 4S outputting 4.2v x 4 = 16.8v when full, but will go down to let's say 3.7v x 4 = 15v near empty... the computer will consume more current when voltage is lower, so the battery must be rated for more than that amount of current.You should also look into the efficiency of the motherboard with various input voltages.Some mosfets or power stages (used in VRM for the processor and RAM) are much more efficient with lower voltages ... the smaller the difference between input and output voltage, the more efficent ... so you may find out that the vrm is maybe 2-3-5% more efficient with 12v than with 19v and using a 12v battery may extend the life by minutes.You may be able to go even lower, but there's often some components like let's say mosfet drivers which may need a high enough voltage, like let's say at least 8-10v to operate.The minimum of 12v may be a requirement just because the motherboard has fan headers with 12v in them and probably sata power connectors with 12v pins for mechanical drives and the dc-dc converter on motherboard is most likely buck only (only reduces 12v..19v to 12v) but if you use sata SSDs you don't need 12v and fans can work just fine at 9-10v (but at reduced speed)

author: eeguy
author_rank: Regular Contributor
timestamp: June 02, 2020, 01:28:43 am
content: Quote from: mariush on May 31, 2020, 06:58:34 amThanks. Is there any good power meter or current sensor that can record the data over a user specified period of time and store the data in a file? The ones listed seem to only store the last instantaneous reading.

author: mariush
author_rank: Super Contributor
timestamp: June 02, 2020, 04:23:58 am
content: There are some power meters with data logging, but it would be probably cheaper to get a clampmeter or digital multimeter with data logging.With a clampmeter, you can just split the 3 wires in the power cable going to pc, and place the live wire inside the meter clamp and you'll get the current flowing through the wire.With a multimeter, you'd have to place the probes in series with your power supply, between the ac cable and the power supply - easiest would be to open the power supply and desolder the live wire going to the IEC connector, and place your probes between that IEC connector and the wire going to the power supply.In both cases, you can measure the current and just multiply it with voltage (which will fluctuate a bit, but you can sort of average it) to get your power, how much the pc consumes.

author: eeguy
author_rank: Regular Contributor
timestamp: June 02, 2020, 12:12:58 pm
content: Quote from: mariush on June 02, 2020, 04:23:58 amThanks. What is the different between clampmeter and power meter? If accuracy and ability to data log is more important than cost, which product/model do you recommend?By power cable going to the PC, you mean the one between the AC adapter which came with the PC and the PC itself? What is "live wire"?

author: mariush
author_rank: Super Contributor
timestamp: June 02, 2020, 03:37:16 pm
content: The power cable that goes to your power supply has 3 wires inside:1. Live - the wire that brings electricity into your computer2. Neutral - the wire through which electricity returns after going through the power supply3. Earth - grounding, to protect you from electric shocksThis is very simplified and a bit wrong, but it's good enough for a total beginner.A clampmeter is a multimeter with a loop ... if you place a wire through the loop, the meter can measure the flow of electricity through the wire. So, you don't have to cut the wire to put the meter between the two cut pieces of wire to measure the flow of electricity.However, if you just place the whole power cable into the loop of the clampmeter, you will not measure anything relevant, because the meter will measure the current going one direction through the live wire, and the current going in the other direction through the neutral wire, so when those two add up, you'll get a value close to 0, if not actually 0.With a regular multimeter (or the probes of a clampmeter, because some clampmeters also work as regular multimeters) , you have to tap into the live wire or the neutral wire to measure the current flow. Doesn't really matter which one, the only difference will be one will show you a negative number on the screen.Here's another very informative video about clampmeter vs multimeter:Here's a review with explanation of a cheap and good clampmeter which explains how it works and does example measurements :