sim_tcr
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12V 7.2Ah lead acid battery charging question.

Wed Mar 13, 2013 7:26 am

Hello,

My Raspberry Pi is hooked to a 12V 7.2Ah lead acid battery (from the +ve and -ve terminals of battery) via a dc to dc adjustable regulator and it works fine.
Battery is from an old APC Back-UPS 500 .The connectors from the ups to battery can be easily connected to charge the battery.
Now at the same time Raspberry Pi is running can I connect battery charging connectors to the +ve and -ve terminals of battery directly?
Without using the UPS, are there any other easy solutions to charge the battery. Such as use a 12V AC-DC adapter?

Thanks,
Simon Mandy
http://raspisimon.no-ip.org
Raspberry Pi Model B x 2, Raspberry Pi 2 x 2, Transcend 32GB Class 10, Transcend 16GB Class 10, Transcend 8GB Class 4, Custom 12V 1.5A (stepped down to 5.5V)

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aTao
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Re: 12V 7.2Ah lead acid battery charging question.

Wed Mar 13, 2013 7:45 am

For charging details you really should have a datasheet for the battery. There will be limits on charger output voltage and charge current.
If (and only if) the max charge current is greater than the current drawn by the RPi AND the RPi DC->DC converter can handle the max charge voltage of the battery THEN you can just hook up a charger and nothing will blow immediately.
The biggest problem you face is detecting a full charge. Since the RPi will be drawing current you have no way of knowing how much current the battery is using to charge, it is often when this current drops significantly that the charger "knows" when to stop. Over charging a battery is one way to break it.
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sim_tcr
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Re: 12V 7.2Ah lead acid battery charging question.

Wed Mar 13, 2013 8:23 am

aTao wrote:If (and only if) the max charge current is greater than the current drawn by the RPi AND the RPi DC->DC converter can handle the max charge voltage of the battery THEN you can just hook up a charger and nothing will blow immediately.
LM2596S DC to DC Step Down Adjustable Power Supply Module is connected in between battery and RPi. And as per its specs, Input voltage: 4-35V, Output Voltage: 1.23-30V (I have set output voltage to 5.4V) and Output current: rated current 2A. So is it safe to connect APC UPS charging leads to battery?
aTao wrote:The biggest problem you face is detecting a full charge. Since the RPi will be drawing current you have no way of knowing how much current the battery is using to charge, it is often when this current drops significantly that the charger "knows" when to stop. Over charging a battery is one way to break it.
As I mentioned in my original post, battery is from a APC Back-UPS ES 500. So if i use the same APC UPS battery connector to connect the battery. APC UPS should know when battery voltage drops and when it is full and stop. Am I right?
http://raspisimon.no-ip.org
Raspberry Pi Model B x 2, Raspberry Pi 2 x 2, Transcend 32GB Class 10, Transcend 16GB Class 10, Transcend 8GB Class 4, Custom 12V 1.5A (stepped down to 5.5V)

aged_geek
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Re: 12V 7.2Ah lead acid battery charging question.

Wed Mar 13, 2013 9:55 am

I take it the DC-AC inverter in the original UPS only get switched on when the AC mains power is disconnected?

sim_tcr
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Re: 12V 7.2Ah lead acid battery charging question.

Thu Mar 14, 2013 4:15 am

I have connected by APC UPS' battery charging leads to battery at the sametime running by raspberry pi directly from the battery through dc to dc step down converter. NO issues to report so far.
http://raspisimon.no-ip.org
Raspberry Pi Model B x 2, Raspberry Pi 2 x 2, Transcend 32GB Class 10, Transcend 16GB Class 10, Transcend 8GB Class 4, Custom 12V 1.5A (stepped down to 5.5V)

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aTao
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Re: 12V 7.2Ah lead acid battery charging question.

Thu Mar 14, 2013 7:14 am

sim_tcr wrote:
aTao wrote:If (and only if) the max charge current is greater than the current drawn by the RPi AND the RPi DC->DC converter can handle the max charge voltage of the battery THEN you can just hook up a charger and nothing will blow immediately.
LM2596S DC to DC Step Down Adjustable Power Supply Module is connected in between battery and RPi. And as per its specs, Input voltage: 4-35V, Output Voltage: 1.23-30V (I have set output voltage to 5.4V) and Output current: rated current 2A. So is it safe to connect APC UPS charging leads to battery?
Yes, if the charger can safely supply max battery charge current plus 2A
aTao wrote:The biggest problem you face is detecting a full charge. Since the RPi will be drawing current you have no way of knowing how much current the battery is using to charge, it is often when this current drops significantly that the charger "knows" when to stop. Over charging a battery is one way to break it.
As I mentioned in my original post, battery is from a APC Back-UPS ES 500. So if i use the same APC UPS battery connector to connect the battery. APC UPS should know when battery voltage drops and when it is full and stop. Am I right?
That all depends on how the charger senses a full battery, it might well simply sense current drawn (actually easier than volt sense). It may simply set a target voltage just above 12V in which case the charge current will drop to a safe trickle for a lead acid battery (fatal for LiIon and LiPo)
aged_geek wrote:I take it the DC-AC inverter in the original UPS only get switched on when the AC mains power is disconnected?
Depends on the UPS.
Some have a small blip at switchover, a brief break in supply and an unsynchronised AC waveform. The fancy ones have a charger, a DC power supply and a permanently running inverter that has a power supply switch on its input. The switch is set by incoming mains sensing.
Normal operation is.
AC in to charger and DC PSU, charger charges battery as required, inverter is powered by DC supply.
When the mains fails the charger looses its supply, the switch then powers the inverter from the battery. At no time is the battery being charged and powering the inverter.
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harani
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 12:42 am

Just out of interest, how long does the 7.2Ah battery run your Pi for ?

Glen

Thaddy
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 2:21 am

That depends: If you run it "headless" probably more than two days. If you underclock it probably longer. With a lot of gadgets connected w/o powered usb probably MUCH shorter but around 14 - 15 hours given a max draw of 500 mAh on the USB. The sum is easy, but practice makes it a guesstimate.

BeeryChisels
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 2:35 am

Hello All,
Using the UPS to charge the battery is probably ok, and depending on the configuration it may just look after itself. If there is a blip at changeover, then it may not.
When looking for a full charge, the system will be looking for a drop in current, not voltage. It may look for a low voltage in order to start charging again.
As a general rule, don't use car-type battery chargers, as the output is more than likely to be half- or full-wave rectified AC with no smoothing. Readings taken using ordinary meters may be misleading, with or without a battery connected, as the peak voltage may well be higher than that shown. The Pi won't like that, and depending on the peak voltage, some regulators might not like it either!
If you obtain a cheap A-D converter, you could rig it to the GPIO port so that the Pi can monitor its own supply voltage. You could display the waveform on the VDU, a sample rate of 1,000 per second should do the trick, although a bit faster would be nice if you suspect changeover glitches.

sim_tcr
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 4:24 am

First two days I left the pi running and battery charging at the sametime.
Next day, I switched the main off and left the pi running. After 2 days pi died. When checked battery voltage, it was below 5 volts.
I tried charging the battery back using the UPS. Its been more than 3 weeks now, still charge has not reached 10V. :-)
I guess draining the battery (bringing down the voltage less than a particular voltage) killed the battery.
I dont know how to recharge the battery now.
http://raspisimon.no-ip.org
Raspberry Pi Model B x 2, Raspberry Pi 2 x 2, Transcend 32GB Class 10, Transcend 16GB Class 10, Transcend 8GB Class 4, Custom 12V 1.5A (stepped down to 5.5V)

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Burngate
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 11:09 am

A 12v 7.2Ah lead-acid battery has 6 cells, each with 2v nominal and about 7.2Ah, but they won't be identical

Discharging it, one cell will go flat before the rest. After that, the rest of the cells are forcing current through it, trying to charge it backwards. It won't like it.
In fact you may have done that to three of the cells, but only one is now a short-circuit.

In future try not to discharge it to the point where the total voltage is one cell less than full nominal voltage - in this case 10v

BeeryChisels
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 1:26 pm

As the battery voltage is now so low, the UPS might regard it as a short circuit and not even attempt to charge it because its protection circuitry will have activated.
In this instance, you could try a car-type battery charger, but DO MAKE SURE that NOTHING ELSE is connected to the battery or the charger while doing this. Some of the clever ones that are often used on motor-cycle batteries can recover batteries which have been deep-discharged.
Measure the voltage before connecting to the charger. Leave it charging for a maximum of five minutes and check that the charger is not overheating. If it is, give up now before you burn the house down.
Otherwise, disconnect the battery, and see if the voltage has increased. It may not be by very much. If there is no increase, give it another five minutes. If there is still no increase, give up now.
One question: Is this a gel battery, or does it contain liquid electrolyte? Gel cells are very sensitive to being deep-discharged, and rarely recover.
Good luck!

cleverca22
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 1:42 pm

sounds like you also need proper cut-off systems, to cut power if you go below 10volts

W. H. Heydt
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 4:22 pm

One point that no one else has mentioned... OP said the output of his DC-DC voltage regulator is set to 5.4v. That's a bit too high to be a good idea. The Pi is supposed to get 5v+/- 0.25v. Now, you may be seeing a drop to 5.25v if you're using a low quality USB lead, but you can't rely on that. The DC-DC should be set no higher than 5.25v.

sim_tcr
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 4:28 pm

W. H. Heydt wrote:One point that no one else has mentioned... OP said the output of his DC-DC voltage regulator is set to 5.4v. That's a bit too high to be a good idea. The Pi is supposed to get 5v+/- 0.25v. Now, you may be seeing a drop to 5.25v if you're using a low quality USB lead, but you can't rely on that. The DC-DC should be set no higher than 5.25v.
When I set the voltage as 5.25v actual voltage pi shows is 4.7V (with no load). If I set the voltage as 5.4V pi shows voltage as 4.99V.
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MrEngman
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Apr 14, 2013 7:42 pm

W. H. Heydt wrote:One point that no one else has mentioned... OP said the output of his DC-DC voltage regulator is set to 5.4v. That's a bit too high to be a good idea. The Pi is supposed to get 5v+/- 0.25v. Now, you may be seeing a drop to 5.25v if you're using a low quality USB lead, but you can't rely on that. The DC-DC should be set no higher than 5.25v.
Absolutely nothing wrong with adjusting the PSU voltage so the voltage at the Pi test points is at 5.00v. I think it is actually quite a good idea. Obviously the higher the quality the power cable is to reduce the voltage drop in the cable and fluctuations in voltage as current consumption varies the better but the voltage at the Pi test points is the important measurement.

I also have the output voltage on my PSU currently set to 5.4v. The voltage at the PI test points is 5.00v when the Pi is idle. My power cable drops about 0.27v and the fuse on the Pi about 0.13v.

I have a wifi dongle connected to one of the USB ports and a hub for the keyboard and mouse connected to the other USB port. Disconnecting the wifi and hub and the voltage on the Pi rises to approx. 5.03v, well within the recommended operating spec. With the wifi and hub connected and the wifi busy the voltage on the Pi drops to about 4.95v or a little less occasionally.

Generally if I change the power cable I will adjust the PSU output voltage to get 5v at the Pi test points. However, I've just checked a much higher quality cable, normally used with my USB DVD writer, with my PSU still set to 5.4v and the voltage at the Pi test points is still within spec at 5.20v max. with no peripheral devices connected. If I used this permanently I would adjust the PSU output to get the voltage at the Pi tests points back to 5.00v but as my current configuration has been running quite happily now for getting on for a year I'm not going to change it now.

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nosretep
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Dec 22, 2013 8:26 am

cleverca22 wrote:sounds like you also need proper cut-off systems, to cut power if you go below 10volts
Would two LM2596S perform this task?

RaspPi <--> LM2596S set at 5 Volts <--> LM2596S set at 10 Volts <--> 12 Volt battery

The first LM2596S ensures that the RaspPi is getting 5 Volts
The second LM2596S ensures that the 12 Volt battery isn't being drained

I'm very new to this, please (anyone) let me know how off base I am.

Thanks!

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Tage
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Re: 12V 7.2Ah lead acid battery charging question.

Sun Dec 22, 2013 4:38 pm

nosretep wrote:
cleverca22 wrote:sounds like you also need proper cut-off systems, to cut power if you go below 10volts
Would two LM2596S perform this task?

RaspPi <--> LM2596S set at 5 Volts <--> LM2596S set at 10 Volts <--> 12 Volt battery

The first LM2596S ensures that the RaspPi is getting 5 Volts
The second LM2596S ensures that the 12 Volt battery isn't being drained

I'm very new to this, please (anyone) let me know how off base I am.

Thanks!
this is not a good idea. you need to set up your system so that the load on the battery is disconnected if the battery voltage drops below 11.0V.
if you have another power source available you could build a charger. the continuous voltage (applied for more than a day) should not exceed 13.6V. to charge the battery you need to apply 14.4V, but it is not good to keep the battery voltage at this high level when the battery is fully charged. you could have a timer that when it is running keeps the charge voltage at 14.4V and reduces the charge voltage to 13.6V after ten hours or so. these voltage values are for SLA batteries at room temperature, SLA = sealed lead acid batteries.

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