# Portable power pack

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#### Mark spice

##### New Member
OK so I'm after a bit of wisdom here, I've built a portable power pack for camping, fishing, festivals etc and am struggling to understand the ah and how many charges I should be getting out of it etc so here's a couple of questions I was hoping you guys could help with . .

So its all wired up and working fine I'm using a 12v 7ah sealed lead battery and have a win usb socket and a voltmeter connected up and it charges my phone and ipad fine until it gets to around 8 volts then my phone just stops charges it acknowledges that's it connects to a power supply but won't actually charge?

Also it usb sockets are 5v 1ah and 5v 2.1 ah can anyone explain what this means so I have a better understanding of how long it should charge and how many charges I should be getting from it?

Thanks mark

#### spec

##### Well-Known Member
Hi Mark,

Welcome to ETO

As a precaution, please stop what you are doing until we can investigate and advise further. If you connect any USB equipment to any voltage higher than around 5.2V you stand a good chance of blowing up the equipment (I phone etc).

spec

PS: I see you are from the UK. Which area? Care to put it next to 'Location' on your user page so that it displays in the window at the left of your posts.

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#### Diver300

##### Well-Known Member
I assume that your USB socket / voltmeter is something like this:-http://www.ebay.com/itm/Car-Motorcy...326014?hash=item5686f4033e:g:iEEAAOSwdWBXPDL8

That will have a converter to convert voltages around 12 V to 5 V. When the battery voltage is down to 8 V, you have taken all the useful energy from the battery. That is too far and you risk damaging the battery. If it does get that low, charge it as soon as possible. Even at 11 V, you have taken nearly all the energy from the battery.

At 8 V, the converter that converts around 12 V to 5 V will have stopped working correctly, and the 5 V will have started to drop. That is why the phone won't charge. There isn't much you can do about that, and anyhow all the energy in the battery has been used up.

The sockets are 5 V, 1 A and 5 V, 2.1 A (Not Ah). The 1 A and 2.1 A are simply the maximum current that the sockets are rated to. With USB, it is more complicated as the data pins are used so that the phone etc can work out what type of socket it is connected to.

A 12 V, 7 Ah battery should be able to supply 1 Amp for 7 hours, or 0.1 A for 70 hours. It will probably be less than calculated at higher currents, and more at lower currents.

The 12 V to 5 V converter, if it were perfectly efficient, would produce the same power out as it takes in, so if you had a 1 A load at 5 V, that would be 5 W so it would need 0.417 A from the battery, and that should give you 16.8 hours running from the battery.

Of course, the converter won't be perfectly efficient, and both the voltmeter and the converter are likely to take some current whether there is a load or not, so both of these things will reduce the life. Certainly leaving the voltmeter and converter connected when the sockets aren't in use isn't a good idea. On the other hand, when you are charging a phone, it won't take the full current for the whole time.

Another way of looking at it is that the lead-acid battery is rated at 84 Wh (Watt hours) and a mobile phone battery is rated at around 6 to 8 Wh, so you should be able to charge a phone 10 times, but say 5 or 6 to allow for the losses.

#### Mark spice

##### New Member
Well that has cleared up a lot of doubt in myself there thanks!! And also answered and lot of questioned as to why it's not charging etc . . . Correct I am using a very similar usb socket to the one you have shown,ok show really then I only 2v of usable power before I drop below the 11v and the voltmeter shows 13v when fully charged. My next question would be is there a better device/meter that would show usable power as a pose to just volts? Like a percentage or battery bar?

So I'm assuming if I linked 2 12v batteries together I would be getting 14 ah thus giving me around 10 charges on the safe side allowing for use of the voltmeter (which if I put on a separate switch would save me power if I only turned it on the check what was left in the battery)

#### Heilman Hackatronics

##### New Member
Your power source the sealed lead acid battery is 12v 7ah, this means your battery can push 12 volts at 7 amps for 1 hour.
I am uncertain why the USB sockets have ah ratings, especially since they have different values.

#### Diver300

##### Well-Known Member
If you have two 12 V, 7Ah batteries, yes you can link them in parallel to give 14 Ah.

A separate switch for the voltmeter wouldn't hurt. If you have a multimeter, you could measure how much current the voltmeter takes to see if that makes a difference. If you don't know how to measure current, ask or look for tutorials.

You should charge each battery separately before linking them in parallel, so that they are at close to the same voltage when you link them. A fuse in series with each battery is a good idea, as you can burn wires quite easily with batteries like that.

#### Mark spice

##### New Member
OK I will look for video or wiring diagram for linking in parallel thanks, because I am putting the batteries in a sealed box is it possible to charge them whilst they are linked together is the box?
And I will check as I have a multi meter I never bothered beforehand as I assumed the drain would be minimal

#### Mark spice

##### New Member
Also I have mine set up with a single battery at the moment with a charging port on it the trouble is when I put it on charge the voltmeter jumps straight to 12.7 so the only was I know ow if it's charged or not it to unplug the charger every few hours and see if the voltmeter stays the same, is there another way of wiring so that the meter only goes up and the batteries charge does?

#### Diver300

##### Well-Known Member
You shouldn't put lead acid batteries in a sealed box. They can give out an explosive mix of hydrogen and oxygen, so you need to let that escape, especially when charging.

With lead-acid batteries, voltage does not give much indication of the state of charge, so the battery voltage is what you are seeing. There is no other voltage to measure.

12.7 V seems a bit low to get a lead-acid battery fully charged.

#### dr pepper

##### Well-Known Member
The usb sockets are not amp hour rated, just amps, one socket will supply 1a and the other 2.1a, some devices will charge at a higher current, some tablets use 2a.
To calc the current from usb to 12v divide the usb current by 2.4 (12v/5v), so 1a @ 5v = 416ma (0.416a), 2.1a = 875ma (0.875a), this is assuming that the converter is 100% efficient, it will be more like 85-90%.
If you were charging your 'phone at 1a then your battery would last about 12 hours or so, maybe a bit less as most manufacturers are optimistic on the ah capacity.

Another way to do it is pull the battery from your 'phone look at the ah rating, multiply it by its voltage rating this gives you watt hours, then do the same with the 12v 7ah battery (84wh) then divide this by the phone battery w/h and then subtract 20% or so for the losses, the result being how many times you can charge the phone from the 12v battery.

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#### Mark spice

##### New Member
Oh I defiantly be taking them out the box to charge! But still is it possible to charge them both together or am I better off buy a 12v battery with higher ah?
And is there an accurate way of measuring what's left as the voltmeter is a bit useless?

#### Diver300

##### Well-Known Member
You can charge both together. That is no problem.

There isn't any accurate way of measuring charge.

#### k7elp60

##### Active Member
Lead acid gel cell battery's AH capacity is determined by the 20 hour rate. A 7AH/20=.35amps of discharge. If you are discharging it at 7A it will discharge in about 30 minuets. Yes you can put them in parallel, if they have the same date code when you bought them and they both have been discharged similar amounts.
If that is not true then when you connect them in parallel one is going to discharge the other. The best way to parallel ones that are not the same is use a diode on the output of each battery to a common point. In other words connect the cathodes together and connect each anode to a separate battery.

#### k7elp60

##### Active Member
You shouldn't put lead acid batteries in a sealed box. They can give out an explosive mix of hydrogen and oxygen, so you need to let that escape, especially when charging.

With lead-acid batteries, voltage does not give much indication of the state of charge, so the battery voltage is what you are seeing. There is no other voltage to measure.

12.7 V seems a bit low to get a lead-acid battery fully charged.
It depends upon the age of the battery and the voltage check time after a charge.

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