Pass through power bank

I think the issue is that once the battery is fully charged the switch to constant voltage charging may reach a point where the battery is being discharged. I assume if the voltage starts to droop the charger will revert to constant current mode, etc. switching back and forth, charging and discharging the internal battery. Such light charge cycles will eventually wear out the battery sooner than if it were managed more like a "proper" USP. That would imply that there is a charging circuit for the battery and a separate supply for the output which does not draw from (and disrupt) the operation of the charging circuit.

The documents on the Anker web site support this. The lighter 10,000 mAHr unit says it will be damaged if used this way and the larger unit mentioned here says it "may" be damaged.

--

Rick
Reply to
rickman
Loading thread data ...

That seems quite likely. There's certainly some missing information. There's nothing specific on the Amazon site advertising the 15000 mAh battery. About all I could see is that both USB sockets have the same label alongside them and IIRC there's a comment about charging two devices at once.

Methinks this could be a case where an SLA may be the better solution simply because you can use a much simpler charger. Building one that switched between 'fast charge' and 'float' (or even ON and OFF) on voltage should be simple enough. I notice that COTS UPSes don't seem to be in a hurry to switch from SLA to to LiPO chemistry.

--
martin@   | Martin Gregorie 
gregorie. | Essex, UK 
 Click to see the full signature
Reply to
Martin Gregorie

Funny, my need for a UPS for my pi is because when the Internet connection is flaky and needs to be reset, I cycle the UPS that powers my entire bench! lol I want the pi to not go down when I do that just like my laptop. I might be able to do this with a super cap on the 5 volt line...

--

Rick
Reply to
rickman

For that sort of job, I'd probably use 4 AA size NiMH or hybrid cells on the end of a LM358 plus transistor constant current source set to feed them 1% of capacity + RPi current draw and, if I was feeling keen, a 7805 or equivalent to drive the RPi. The point being that NiMH last forever if trickle charged at 1% of the rated capacity, i.e. 10mA into a 1000mAh NiMH.

Yeah, I know you prolly think I'm a heather, but I've used this sort of rig for long-term NiCd and NiMH maintenance over the last couple of decades and never had a problem with any of them.

--
martin@   | Martin Gregorie 
gregorie. | Essex, UK 
 Click to see the full signature
Reply to
Martin Gregorie

What's a heather?

--

Rick
Reply to
rickman

Wild-assed guess, for laughs, follower of Heath-Robinson, the British Rube Goldberg?

Mel.

Reply to
Mel Wilson

Heathen?

Reply to
Tony van der Hoff

For 'heather' read 'heathen'.

--
martin@   | Martin Gregorie 
gregorie. | Essex, UK 
 Click to see the full signature
Reply to
Martin Gregorie

Yep. Uncaught typo. Though maybe I should have said 'electronic heathen'.

--
martin@   | Martin Gregorie 
gregorie. | Essex, UK 
 Click to see the full signature
Reply to
Martin Gregorie

formatting link
, obviously.

Reply to
A. Dumas

Agree, or even a sealed lead acid battery with a DC-DC converter to 5v (easily available in the form of cigarette light socket mobile chargers).

Lithium batteries have special properties making them ideal for portable use, but that doesn't apply here. They are much more complex to charge, and absolutely mustn't be continuously trickle or float charged. That makes them unsuitable for use in a UPS of the type discussed here, unless there was more complex circuitry designed in to the UPS to take them out of the circuit once charged. It seems the products people have identified are not designed this way, which is probably why the manufacturers don't support simulteneous charge and use.

The easiest way to build one would be a 12V SLA battery and charger designed to float at 13.8V, and a DC-DC converter to supply 5V for the Pi. (These are available at 2.1A nowadays, and even 2 x 2.1A.) You could add some circuitry to turn off the load if voltage drops to

10V (to protect the SLA battery), and if you expect the battery to be run down to this level, use a deep discharge battery, which is capable of deep draining without seriously shortening its life. Battery capacity will typically decline linearly to nothing over about 5 years in float charging at 13.8V - use that to decide how often to replace the battery. Sizing the battery and charge current capacity depends on the load and the min hold-up time you need after a power outage. Another factor is how long you are prepared to wait for the battery to fully recharge after power restoration before you have enough energy stored to satisfy your min hold-up requirement again - you have an at-risk time when the UPS will not yet be able to meet your hold-up requirement.

With batteries which need constant current charging (or controlled current charging) such as NiCd/NiHM/LiPo, you need a different approach.

--
Andrew Gabriel 
[email address is not usable -- followup in the newsgroup]
Reply to
Andrew Gabriel

Nope, never seen *that* definition before.

--
martin@   | Martin Gregorie 
gregorie. | Essex, UK 
 Click to see the full signature
Reply to
Martin Gregorie

I thought about suggesting that, but its fairly hard to find 6 or 12v SLAs of under, say, 2.5AH and anything bigger is likely to be overkill for running an RPi, Also, looking through the Sunpower catalogue, I didn't find any multistage chargers for 6v SLAs. However, if you use, say, a 7.2Ah 12v SLA I agree its dead easy: connect a 2-stage charger to one side and use a cigarette socket 2v->5v converter to provide the 5v.

Again, I've done this for some years. I use a pair of 12v 7.2AH SLAs to run the instruments in a glider and have a pair of Sunpower 2-stage chargers to keep them topped up at home. Most glider instruments want

12v, but the PNA I use for navigation wants 5v via a mini-USB socket, so I use a cannibalised cigarette socket 12v->5v converter to power it. Yes, the PNA has an internal battery, but that's only good for 2-2.5 hours and a good cross country flight is 4 hours plus. 'Cannibalised' because you get more reliable connections by removing the converter's plastic case, installing the PCB in a small Maplins metal box and soldering the 12v inputs to it. Stuffing the unmodified converter into a Maplins/Farnell/RS car lighter socket is a recipe for bad connections: all those lighter sockets are just rubbish for anything except heating a lighter. For home use, mounting the converter in a plastic box would probably be OK, but these (tiny, dirt cheap) converters are switch-mode devices that put out a lot of RF hash: not good to have anywhere near an airband radio set.

Mine is a Compaq iPAQ in-car charger that I bought off eBay in 2004 and is rated at 1.4 amps.

Nice touch - a job for one of the 8 pin PICAXEs?

They deteriorate anyway. My 7.2AH glider batteries are always put on charge after a day's flying and never left on charge for more than 12 hours. My panel draws less than 1A at worst case (measured with everything on and the radio on receive at 400mA). I cycle the batteries once a year to check capacity and notice that even good quality Yuasa batteries only hold full capacity for the first three years and then decline quite steeply (>20% a year) after that. I've seen 500 cycles quoted but don't think its that much in real life.

--
martin@   | Martin Gregorie 
gregorie. | Essex, UK 
 Click to see the full signature
Reply to
Martin Gregorie

That was my thought too but it's one hell of a typo. :-)

--
J B Good
Reply to
Johny B Good

...confirming that he meant "heathen". :-)

--
J B Good
Reply to
Johny B Good

I'm surprised that SLA batteries die so quickly. I guess they are rather different from the lead acid battery used in automotive apps where they can last much longer. I worked for the railroad once and saw batteries that were 20 years old still working fine. They were backup for crossing signals and were on trickle charge 99.9% of the time.

--

Rick
Reply to
rickman

Yes, surprised me too. My pnly consoation is that other brands I tried (uniRoss, Maplins one brand) were on their way down after two years.

The ones I use are SLAs (gel cells or GF-packed electrolyte spaces I don't know if that makes a difference to their lifetimes. It makes a big difference to usability: the glider's battery box was designed round a single wide, flat 12v battery of unknown capacity that's hard/expensive to find now, but i can get two 12v 7.2 AH batteries into the same space if I stand them on end and raised the lid 50mm. Obviously this is only possible because they are sealed types. And, in any case they spend at least 80% of their time the right way up being charged or waiting for flying weather.

--
martin@   | Martin Gregorie 
gregorie. | Essex, UK 
 Click to see the full signature
Reply to
Martin Gregorie

The big problem of keeping SLAs on permanent float charge, typically the case for UPSes and _anything_ that acts as a UPS, is avoiding excess float voltage across any one cell in the string (6 cells in the case of 12v lead acid batteries - or 24 cells in total for the battery pack used by my venerable SmartUPS2000, currently bypassed until I can get hold of a suitable set of batteries at a seriouly low price).

Getting the float charge voltage per cell on each and every cell in the string can be nigh on impossible, especially when using multiples of 12v batteries to provide the higher voltages (24, 36 and 48 volts).

The 2.3v per cell float charging voltage is a compromise between sulphation and corrosion. Too low and you risk sulphation. Too high and you increase the rate of corrosion (excessive voltage also increases the risk of water loss).

To further complicate the 'ideal voltage' level, this is also effected by temperature which, according wikipedia, requires a

-0.0235v adjustment for each deg C increase above the 20 deg C reference standard for a 6 cell (12v) battery.

You might think that the typical 5 year service life of an SLA battery pack in UPS service is unimpressive but, if you care to substitute the UPS SLAs with car batteries, you'll swiftly change your mind (usually after a mere six months of continous float charge and a small fortune in de-ionised water).

I know this only too well from experience, three experiments in total! The first 'experiment' was keeping a single 12v car battery charged from a 13.8v CB powerpack to allow not only battery backup but also to serve the high current demands of a "Burner". I didn't realise this actually was an experiment at the time. It took the 20/20 vision of hindsight to reach this conclusion over 20 years later. At the time, I simply put this down to the 'bad luck' of using a defective battery.

When I got my hands on the SmartUPS2000 at a radioham rally about a decade back, it didn't have the seperate clip on battery box so I had to use an external battery pack. I think I used a pack of four 7AH

12v SLAs to begin with. Since the orginals specified 17AH SLAs, I investigated a cheaper option in the form of a set of 36AH car batteries for a mere 60 quid from a local automotive dealer.

Initial testing results with these bad boys wired in parallel with the SLAs were very encouraging indeed but the batteries gradually got more and more 'thirsty' over the following months before developing low voltage cells due to excessive self discharge effects which, effectively, rendered the whole battery pack useless after only 6 months or so.

By then, I'd managed to accumulate a complete set of 24AH SLAs to go with the 7AH ones. I didn't rush back out to my friendly automotive dealer straight away for yet another battery bargain, that second experiment didn't happen until maybe 6 to 12 months later (the penny _still_ hadn't dropped that I had run 'The Experiment' for a second time). Extremely annoyingly, the 'new' set of car batteries suffered exactly the same fate 6 months or so later.

Only then did it occur to me that in spite of the more 'benign charging regime', a total lack of vibration and extreme variations of temperature such batteries seemed well able to cope with, there was a fundamental problem in using such batteries as UPS backup power.

It was only then that, like Milton's frisbee, it struck me! I finally realised that I had experienced exactly the same problem more than two decades ago with that aforementioned battery and CB 'charger/PSU' setup. Only then did I shy away from "The Cheap Car Battery Solution".

I strongly suspect the main problem was the sustained float charging voltage that was causing corrosion in the paste filled plates and I am tempted to experiment again, only this time reduce the 13.8v per 12v battery down to 13.5v. I'd need to find another set of car batteries at a bargain price yet again before investing in a fourth experiment though.

Another alternative would be to try a set of deep cycle leisure/marine batteries (again, only if I can find a trader prepared to sell me them at a bargain price) since these are designed for similar service only with more frequent deep discharge cycles in mind.

In this case, a Raspberry Pi with a low power demand, the ideal battery voltage is 2.3v (float) i.e a single lead acid cell. A constant volt charger can guarantee that all the cells are getting the right voltage (there's only the one cell to worry about and any additional cells wired in parallel will also enjoy this benefit).

You do need a high efficiency low input voltage converter to generate the required 5 volt supply in this setup but if it avoids the conditions that would otherwise accelerate corrosion due to cell voltage imbalance issues common to 6 and 12 volt lead acid batteries, it will simplify battery care (only the one cell to check voltage (and keep topped up if it's an open cell type)).

What you lose in voltage, you simply make up in ampere hours to get the same watt hour capacity. One bonus of packing all your watt hour requirements into a single cell is that it will either occupy slightly less space than the equivilent in a 6 or 12 volt battery or allow you to gain a larger watt hour rating out of the same volume (but slightly heavier) battery with this gain being greater over the 12 volt battery than over the 6 volt battery options.

In this case, it's worth giving the single cell SLA option serious consideration, after all, you're going to need a 5v switching converter in the mix anyway so why not make it an up converter?

The only remaining problem lies with obtaining a high efficiency single cell mains charger. The standard shotcky rectifier diodes used on 5 volt supplies represent a 10% loss of efficiency all on their own from their typical 450 to 500mV forward volt drops when used in the classic bi-phase fullwave rectifier mode at full rated output current.

The only way to eliminate such losses in a 2.3v charging supply would be the use of power FETs in an active rectifier circuit. In theory, reducing the volt drop to 100mV or less, depending on output current and the choice of and number of FETs used. Such extra complication will no doubt increase manufacturing costs and hence price _and_ availability. I've no doubt that the benefit of large scale manufacturing will reduce the premium to just a few percent but that pre-supposes a high market demand.

More realistically, a compromise on this 'ideal' would be to choose a

14AH 6v SLA over a 7A 12v SLA. They should both cost about the same (same materials and manufacturing costs in either case, ignoring disparities in market demand between these two options). Fewer permutations by which cell voltage imbalance can start the cascade of overcharging each cell in turn assuming you don't employ them as part of a higher voltage string.

Perhaps a better version of the 6 volt SLA, where portability is not a concern, would be to make up the 6 volt battery using seperate cells. This neatly solves both the inexpensive high efficiency charger and voltage converter issues and allows monitoring of the individual cell voltages or, better yet, provision of a voltage sensitive shunt module across each cell to save healthy cells from the effect of being overvolted by other less considerate cells dropping in voltage to due a slightly higher self discharge effect.

Such voltage shunt modules will, naturally include an indicator to show excessive shunt current demand so you can deal with the real culprit in a more timely fashion.

HTH

--
J B Good
Reply to
Johny B Good

Thanks for that!

Stupidly, it had never occurred to me that multicell SLA's could from thje same need for a balancing charger that all multicell LIPOs require. Presumably one could design a 2/3 stage SLA charger that uses a LiPO charger's cell balancing circuitry to work with a set of single cell SLAs?

--
martin@   | Martin Gregorie 
gregorie. | Essex, UK 
 Click to see the full signature
Reply to
Martin Gregorie

I don't know what the reason for using SLA batteries in your glider is, but wouldn't Li-ion be a lot lighter for the same capacity? I think that is the main reason why they use them in autos. I take it this is a fairly sizable glider? What's the wing span? Are you talking about one that carries people? I was picturing a drone type aircraft.

--

Rick
Reply to
rickman

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.