AC Battery Bank (Cont)

[Wolf585]

It my last post I was very new to hobby electronics ( and still kind of am), but I think I've learned a few things after watching a bunch of videos about this.

Previously I was wanting to make a battery bank with an AC output, but I really didn't know how. Basically what I'm thinking now is line some 18650 batteries up in a series of 3 to make around a 12v battery. I could use a bms to make sure it's all charging correctly and stuff, on the other side, I could use one of the  dc-dc converters to keep the current at 12v steadily while a DC-DC Car converter is hooked up to one of the car converts. Do you think this would work or is this just stupid. I wanted to use 18650 batteries because they were pretty small, and stable, put I guess I could use some small lead acid 12v batteries like these: https://www.batterysharks.com/12-Volt-2-9-Amp-Sealed-Lead-Acid-Battery-p/12V-2.9AH_B12-2.9.htm?gclid=Cj0KCQjwx8fOBRD7ARIsAPVq-NkN3tQ9Nv38JuATSJ-z3RZj6AsLvzvY1UweB_jV50Q665s15b94JRMaAtA3EALw_wcB

 

Here's the stuff I was thinking of:

converter: http://ebay.to/2ki11qA

bms: http://amzn.to/2fFTJHX

Edited October 3, 2017 by W-L
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[W-L]

3 minutes ago, Wolf585 said:

-SNIP-

It would depend on what your powering but directly going from DC to DC is best as that would be much more efficient than trying to go from DC to AC and have the device go back to DC. 

[Wolf585]

1 minute ago, W-L said:

It would depend on what your powering but directly going from DC to DC is best as that would be much more efficient than trying to go from DC to AC and have the device go back to DC. 

I'm trying to power my 180w charger for my laptop as well as some other minor camera gear, and multiple phones at once.

[W-L]

1 minute ago, Wolf585 said:

I'm trying to power my 180w charger for my laptop as well as some other minor camera gear, and multiple phones at once.

In what kind of application is this in, the outdoors? If your just needing to top stuff off your would be better off with a variable voltage power bank to keep things powered.

[Wolf585]

4 minutes ago, W-L said:

In what kind of application is this in, the outdoors? If your just needing to top stuff off your would be better off with a variable voltage power bank to keep things powered.

ok, where would I get one of those for a cheapish price?

[unknownmiscreant]

Your system will work, but you are way better off to use a car laptop charger which runs off 12v rather than a DC-AC converter.

 

3x 18650 cells is not alot for this sort of application, so I would recommend going for at least 6 cells. If you want it to be light, go with 18650, but otherwise lead acid batteries like you linked above would cheaper and much heavier.

 

Lead acid cells are also easier to charge. Lithium requires very specific charging conditions to not damage the battery, and can even explode in the case of charger malfunction.

[Hackentosher]

2 hours ago, Wolf585 said:

I'm trying to power my 180w charger for my laptop as well as some other minor camera gear, and multiple phones at once.

Why not get like a 300w inverter?

 

Make sure to get a sin wave one, the crappy square wave inverters make electronics mad. https://www.amazon.com/Go-Power-GP-300-300-Watt-Modified/dp/B00153BH6M/ref=sr_1_8?ie=UTF8&qid=1507003005&sr=8-8&keywords=300w+inverter

[mariushm]

A power inverter will accept a wide range of DC voltage, one that can work with 24v DC nominal (so up to 30-35v for short periods of time) would be ideal.

 

18650 cells have great density (lots of energy in small package) but I'm not sure they're the best solution for you. You definitely need more than just 3, they won't be enough for a laptop that could use up to 180 watts.

 

18650 batteries in general can output up to let's say 3A of current for short periods of time, maybe 2A continuously. 

They have voltage between 3.6v and 4.2v when fully charged, so 3 batteries in series will be able to output for long periods of time 10v to 12v at 2.5A (just taking the middle of those numbers) , or around 25 watts.

The power inverter converting the DC to AC would also be about 85% efficient, so your 25 watts become 20 watts.

Your laptop power brick / charger whatever is rated for 180w but that doesn't mean it always uses 180w or that your laptop consumes 180w all the time. Your laptop will probably only use around 50w when you're in Windows or when watching movies, I suspect you have some big graphics card in the laptop that would require a big part of that rest of the power the brick can provide.

And then, the brick may be so big in order to be able to fully power the laptop with let's say 150 watts, but still have a remainder of around 20-30 watts to slowly charge the built in battery while you game or do stuff with the laptop.

 

Either way, you would have to create multiple "chains" of batteries (for example 3 batteries in series) and then have multiple such packs in parallel.

 

If it's just for some energy at some place, you'd probably be better off with a proper 12v or 24v lead big capacity acid battery - they're heavy but they can give you a good amount of energy for hours, they can be charged fairly easily and they can also power an inverter directly without any fuss.

And you can also find ready made solar panel "managers" which take varying amount of voltage and current from solar panels and convert it to 12v or 24v and shove it inside lead acid batteries.

 

 

[unknownmiscreant]

@Hackentosher, @mariushm Why are you recommending inverters. They are quite inefficient (especially for low loads,) meaning OP will not get alot of run time from the battery pack.

 

Given the cost of an AC inverter (especially a decent pure sine wave one,) I would recommend instead spending this on a laptop charger that is designed to run directly off the cigarette lighter sockets found in cars (If you buy the right one it will also have USB port(s).) This means the 12v from the batteries is still stepped up to 19v, but this is way more efficient than stepping the 12v up to 120VAC only to step the 120VAC back down to 19V.

 

IDK about inveter efficiency, but my knowledge of switch mode supplies and some common sense tells me that when you want to run something off a battery for a long period of time at all costs avoid unnecessarily transforming the voltage. As for the 85% efficiency, I suspect that will be an ideal that is only achieved at a very specific load.

The cigarette lighter charger also has the advantage that is will be a bit smaller/the same size as an inverter, and means that the original AC powered charger does not need to be carried.

 

As for batteries, i would recommend lead acid, as despite lithium having great energy density, battery management is quite complicated, and has fairly severe consequences if you get it wrong. If OP wants something that is small and light, using 18650 cells is a great choice, but you should be aware of the dangers associated with the cells. If you do decide to use them, I would recommend buying a good charger and battery management circuit. Lithium batteries require a very specific constant current and constant voltage charging profile, and can explode if charged over 4.2V. The management circuit is to ensure that power supply to the laptop is shut off before the battery goes too flat, as permanent damage is caused by draining lithium cells below 3.0V. I personally set the cut off at 3.8v for devices I have built to run off lithium batteries. However 3.6v should be fine for your purpose.

 

You should also be aware that lithium batteries loose capacity if they are stored fully charged for long periods of time. That is why all electronics that runs off lithium ships at around 60% charge, or 3.8v at the battery terminals. i would recommend buying a charger that has the option to only charge the battery to storage capacity.

 

I would also say that only 3 cells is a no-no. Whilst it would work, you would be much better off with 6 or 9 cells in 3 banks of 3 (or 2 banks of 3 if using 6 cells.) This is achieved by connecting the batteries so there are 3x 3 cell packs connected in series, and connecting these three packs in parallel.

 

This will provide much better results, as it means that each battery is only supplying 1/3 of the current. The reasoning for this is two fold:

Firstly, higher capacity (3000MAh) cells can only discharge at lower currents. You trade cell capacity directly for max discharge current. Banking up the cells in parallel means that each cell is providing less current, so a higher capacity cell can be used. This is more applicable to higher drain devices, so the effect will not be very significant with an idling laptop, but will be an important consideration if you want to run the laptop at full load.

Secondly, the capacity of batteries drops (usually scarily fast) at higher discharge currents. This means a 3000MAh rated cell can only supply 3000MAh at say 200mA load. If you place a 2A load on it, the capacity may drop to 1500MAh. This drop is battery specific, and the effect is lower with higher quality batteries. You need to look at the discharge curves in the battery data sheet to find out this information.

 

If you choose to use 3 cells, I would recommend a 2000MAh battery, with a higher max discharge current (it will experience less capacity drop under load).

If you choose to use 6 cells, go with a 2500MAh batttery. For 9, go with a 2500-3000MAh battery.

 

But make sure to look at the discharge curves for the cell, NOT just the stated capacity!!! Samsung cells are likely the best, but there are several different models and capacities.

 

If you decide to use lead acid, something around 4-9Ah would be a good choice. I have found sonnenschein batteries to be very good in the past.