"Power Bank" to replace Pb battery?

[Llafurio]

Hi,
a friend with a Cape Cutter on a mooring has to replace his Pb battery. His power consumption requirement is low, mostly fishfinder and GPS, and some VHF. No space on deck to fit solar panels for recharging so needs to recharge at home.

I recommended him to look at these fairly powerful and cheap Li-Ion "Power Banks" widely available today. Has anyone here tried those yet? For instance: https://www.amazon.co.uk/dp/B00YP823NA?tag=aagbp-21&ascsubtag=trd-us-4673929896086962000-20&geniuslink=true . C.

[TimLM]

I use a couple of 20000mah power banks for emergency phone/tablet power. But at 50000mah that is a pretty good back up. As a main power source that sounds pretty good just need to check the amperage available and required. I can't see anything to stop you using them otherwise, nice thought.

[Matthew P]

I have read somewhere (probably an expensive power pack supplier) that cheap power banks have inadequate electronic protection built-in to prevent them overheating and worse, catching fire .  I also know international carriers such are increasingly reluctant to transport lithium power packs, especially by air.

I don't know how much of a genuine concern this is.

Matthew
BR20 Gladys
 

[Llafurio]

Tim, Matthew,
thanks.

I have abandoned the idea in the meantime.

Firstly, I read that these powerbanks have an internal voltage of 3.7 V, and they cascade that electronically up to 12 V, but incurring significant losses on the way. So, even if they say to contain 50,000 mAh @ 3.7 V, that figure may well dwindle to 40,000 mAh or less @ 12V DC.

Then, for the example product linked above, the output at 12V DC is limited to 2.5 A, and that will not power an installed VHF during "Transmit".

Re the fire risk of Li-Ion power banks, I think that is something quite remote, and I would not be too concerned about that. We all carry Li-Ion battery powered devices on us and do not really think twice about the inherent risk of that. My waterproof navigation phone has a 10,000 mAh battery, quite near to a power bank, and I would not leave it behind for the theoretical fire risk.
C.
 

P.S.
I read some more about "mAh". Disappointingly, the seemingly impressive "50,000 mAh" of the quoted power bank, multiplied by the internal voltage of 3.7 V equal an energy content of only 185 Wh, and if that is then cascaded up to 12 V, and counting in the losses incurring on the way, the usable energy content @ 12V is not over 150 Wh. Compare that to the theoretical 240 Wh energy content of the 40 quid 12V 20 Ah Pb battery, and the choice is clear.

[Jonathan Stuart]

If this idea is resurrected then I recommend a self-contained 12v lithium battery. I used a Tracer battery on my BRe (depth sounder, ballast pump plus charging for phones, etc) and took this off for charging when I wasn't on the boat. This more than met my requirements.

https://www.tracerpower.com/tracer-lithium-polymer-battery-packs.html

[Llafurio]

... and there is always the trusted sealed 12 V 20 Ah Pb battery for under 40 quid. Heavier, but still well portable. https://eu.nkon.nl/rechargeable/lead-acid-batteries/12v-vrla/vmf-12v-20ah-loodaccu.html

[Graham W]

If this idea is resurrected then I recommend a self-contained 12v lithium battery. I used a Tracer battery on my BRe (depth sounder, ballast pump plus charging for phones, etc)

Another vote for Tracer, and hang the cost!  They claim to be IP64 splashproof, which (without going into embarrassing details) I can confirm.  They are also very long lived (so far).

One thing that I don’t understand is that despite the dramatic fall in the cost of lithium cells, the price of Tracer batteries seems to have hardly moved in the seven years that I have had mine.

[Paul Beardsell]

... for the example product linked above, the output at 12V DC is limited to 2.5 A, and that will not power an installed VHF during "Transmit". ...

12V x 2.5A = 30 V.A = 30W

Not enough to run a ballast pump.

[Graham W]

I've just used my Tracer 12V 24Ah battery to run my Garmin GPS/fishfinder and Tacktick NMEA transmitter continuously for three days.  I don't know how accurate the battery's inbuilt meter is but at the end of that period, it was showing two out of three green bars remaining, with two red ones in reserve. 

My back of the envelope calculations suggest that with a typical combined draw of about 0.7Ah for both machines, the battery ought to keep going for around 35 hours.  A few hours of recharging in the Kentish sunshine using a 60W solar panel and it was back up to full capacity.

[Paul Beardsell]

12 V x 24 A.hr x 3600 secs/hr = 1036800 J ~= 1,000,000 J
3 days = 259200 secs ~= 260,000 secs

Energy available per second (J / sec = W) over 3 days
= 1,000,000 J / 260,000 secs ~= 4 W

4 W at 12 V = 4 / 12 = 0.33 A

[Alternatively: 24 A.hr over 3 days = 24 / 72 = 0.33 A]
[0.33 A at 12 V = 4 W]

But given the accuracy / veracity of the manufacturers' claims it matters not the underlying technology: 24 A.hr at 12 V is the same Pb or Li. Battery weight is a factor if you're having to carry it about for recharging but not really, otherwise, in a boat. Number of recharges per capital cost, resilience to repeated deep discharge, together with veracity of manufacturer's Ah claim, that's the money issue.

Except that for the power bank referenced initially above, the max output is 2.5A at 12V or 30W which means nothing more than 30W can be drawn at any time from the power bank.

[Martijn]

24 A.hr at 12 V is the same Pb or Li.

Not quite. For a lithium battery it is ok to to have a state of discharge of 0% without causing damage, most lead acid batteries will get irreversibly damaged when the state of charge gets lower than 50%. (30% for AGM batteries)
So a 24Ah lithium battery will give you roughly twice the amount of energy compared to a 24Ah lead acid version.

[Martijn]

Not to mention that Lithium batteries frequently have higher voltages.
24A x 14.2V (LifePo4 battery) equals 340.8 VA
24A x 12.6V (Lead acid) equals 302.4 VA

Again the Lithium battery wins, it is not merely a matter of weight and price.

[Paul Beardsell]

A 20Ah battery isn't a 20Ah battery unless it can deliver 20Ah. It is true that lead acid batteries are damaged by a discharge which is too deep but if that happens to a "20Ah" lead acid battery after 20Ah has been taken from it then it isn't a 20Ah battery. That one can take a properly labelled 20Ah lead acid battery and destroy it by taking 30Ah from it is true, but it doesn't mean that it doesn't reliably deliver 20Ah. Also, when one's device wants 12V and you deliver 14.2V to it, energy is wasted. Generally speaking, the extra voltage isn't usable by a 12V electronics device, or by a cheap LED driver. Incandescent bulbs will shine brighter (and last only a fraction of the time) on the higher voltage, but are terribly inefficient.

[Martijn]

A 20Ah battery isn't a 20Ah battery unless it can deliver 20Ah

True.

[Graham W]

So a 24Ah lithium battery will give you roughly twice the amount of energy compared to a 24Ah lead acid version.

There are so many advantages to lithium batteries.  The only downside I can think of (apart from the high initial cost) is that towards the end of the discharge cycle, if you’re not paying attention you can get caught out as the battery suddently runs out of charge.  At least with lead batteries you get plenty of warning as they peter out slowly.