Cordless hoover battery to power mount

[cuivenion]

Hi, I recently got a cordless hoover with two lithium-ion batteries. I'm wondering if there's any reason I can't use a buck converter and use the batteries to power my mount? The batteries are 2350 Mah DC, 25.2v, 59.22 watt hours. They won't last that long I imagine, 4-5 hours per battery with a HEQ5 if I'm lucky but that would be enough for me.

[Pompey Monkey]

I can't  think why not.

I've been thinking about re-purposing (or even multi-purposing) an e-bike battery for the same reason. 48 V 840 Wh should keep an HEQ5, TEC cameras, dew-heaters, and all the other bits going for a whole night.

At around £300, they are not the cheapest, but don't  half pack a punch!

[cuivenion]

I think I'll give it a go at some point then. That E-bike battery sounds good!

Edited September 22, 2021 by cuivenion

[wulfrun]

One thing to be cautious of, you must never discharge a LiPo below 2.5V for ANY cell in a pack (and even that is pushing your luck, 3V is safer). If the pack has a built-in battery-management system, you are good to go. It should cut off if discharged to "empty". If it doesn't have such circuitry, I'd advise against it very strongly. Chances are a vaccuum cleaner or bike battery etc does have a management system but I would be finding out for sure first.

Edited September 22, 2021 by wulfrun
typo

[cuivenion]

Good point, I would imagine it has, unless the managment circuitry is somehow included in the hoover and not in the battery. When you use the hoover it just cuts out when the battery runs out. It doesn't get gradually slower like I suspect it would if there was no management system and the voltage was getting lower and lower. The batteries are lithium ion.

Edited September 22, 2021 by cuivenion

[wulfrun]

2 minutes ago, cuivenion said:

Good point, I would imagine it has, unless the managment circuitry is somehow included in the hoover and not in the battery. When you use the hoover it just cuts out when the battery runs out. It doesn't get gradually slower like I suspect it would if there was no management system and the voltage was getting lower and lower. The batteries are lithium ion.

Almost certainly. If the pack has only two contacts (+ and -) then yes, must be integral. If it has multiple contacts, it could be in the device. Lithium ion and LiPo are generally the same thing, I ought to have used the same term you did.

[Carbon Brush]

Danger. Do not take apart a lithium battery stack unless you know what you are doing.
This is a serious warning, not one of those 'no user serviceable parts' or 'ask a grown up' warnings from the safety elves.

I have taken lithium packs apart, and put them together. But I work with this sort of thing for a living so hopefully stand a better chance of survival.
If you inadvertently short circuit a cell or battery pack, the fault current is often high enough to melt the connecting wires, after the insulation has caught fire.
Explosions with lithium fires are not far away.

Do you know for certain the battery pack has a good internal battery management system? If not, stay clear.
Here are two examples.....

Considering first a 'quality' pack. It was from a Dyson hand held. Showing the 'buy a new one' light. So I had to go and take a look out of interest. One cell was iffy.
This pack contained an extensive management circuit, on a cell by cell basis. So there are no worries about individual cells getting over charged, or over discharged.
Just connect a charger (or vacuum cleaner) and the pack looks after itself. Even if the vacuum motor stalls and pulls too much current, the pack just shuts down and restarts when the problem is cleared.
With something like this, you are reasonably safe to re-purpose for astronomy.

Now at the opposite end of the spectrum. A huge lithium pack from a chinese copy of a Segway.
I forget now the size. But it made ebike packs look small. About the size/weight of a small car battery, which puts it about 3x the energy content.
The UK buyer could not get it to charge properly and it was replaced after some argument.
I offered to dismantle the pack to save the owner the hassle of safely delivering this for recycling. Around 60% of the cells were serviceable.
You try recycling big faulty lithium at the local tip. If they realise what it is, they won't let you through the gate - you are carrying a bomb.
There was minimal cell balancing circuitry, meaning normal charge or discharge could damage cells.
No temperature monitoring - which is vital both for safety, and for setting charge voltage.
Lithium cells are very touchy about overcharge, or over discharge. They are easily wrecked.
Over current protection was a crude piece of flat copper to make a fuse.

If in doubt. Buy a 12V Tracer pack, or one of the astro branded lithium packs.

It is possible to repurpose a 12V car jump start pack. But there is significant effort in proving correct charge and you have to add your own discharge protection.

HTH, David.

[cuivenion]

Hi David, message received. I wasn’t going to disassemble the battery, I’m not that brave. The battery seems to have a fairly standard jack connecter, so I was just going to put a buck converter between the battery and the mount using a wired connection. I will double check that the battery has a management system inside before trying it out though.

The hoover is a Shark IZ251UK, bought from a uk retailer. I’ve added a couple of images of the battery if anyone’s curious.

Edited September 22, 2021 by cuivenion

[Carbon Brush]

OK, you are happy using the battery as it stands. How do you use it and what reasonable operation will it give?

Given Shark, I think you can trust them to include a proper BMS (battery management system) within the module.
Is the jack the charge & power out? If so, life is easier. Sometimes batteries have different charge and load connectors.

You will need to take careful measurements of the 'standard' power jack.
These are generally known as 'DC connectors' when you are buying.
Once you depart from the 2.1mm pin connectors used on many lower cost mounts, the idea of a standard goes out of the window.
I would use a caliper gauge to measure the barrel diameter and small twist drills into the socket to gauge pin diameter.
That will allow you to locate a suitable connector.

First of all you need to think about the DC/DC switcher convertor to derive 12V for a mount.
Are you designing yourself? Relying on a £4.99 PCB type? Buying in a convertor module with known name from a reputable supplier?
Your best bet is to buy in a known name. A cheapie PCB may well have high ripple. Or may have a poor design resulting in a toasted mount electronics.
Take note some convertors require a minimum load for good operation - the voltage rising on low load.
Whatever I used, I would add a 15V 5W or similar zener diode to limit the output in the event of a fault. A fried zener costs much less to fix than a fried mount.

Having done all this, you need to think about a suitable mount fixing. Where does the battery sit? Secured to a tripod leg?
Then an enclosure for the DC/DC convertor and something to fix it to the tripod. Probably a flying lead to the battery.
Either a socket on the convertor box, or another flying lead.

Next, what energy will it give?

Lithium cells are almost always rated optimistically. The manufacturer gives a power delivery based on a certain discharge current at a certain temperature.
When you deviate from these, the capacity is reduced. Check cell manufacturer data for details.
The performance hit is similar between good manufacturers. They all use similar chemistry.
What I'm saying is that in the cold, delivering low current, you may not obtain 59Wh. 

What is the efficiency of your DC/DC convertor? Assuming you buy in a named module, the manufacturer will provide data.
The efficiency depends on the load for any convertor. As a general rule if you choose one with a limited input voltage range, it will be a little better.
80% efficiency is a reasonable assumption. 90% if you are lucky.

Taking the above losses into account, Your 59Wh battery is probably going to deliver 40-something watt hours.
Or nearly the equivalent of using a 12V 4Ah lead acid.

To me it seems a lot of bother for a basic result. If doing this, I would start out with a larger capacity battery.
The housing, protection and fixing issues are similar for a larger battery and a 4 cell stack avoids the DC/DC convertor.
Yes if you go really big on cells the design changes.

When everything is considered, the standard mains supplies and power tanks start to look reasonably priced.
But as a bit of construction fun, why not.

HTH, David.

 

 

[cuivenion]

Thanks for the advice David, I'll definitely bear all that in mind. Do you have suggestions for the reputable brands I could buy for the converter please?

[Carbon Brush]

Recom, Murata, Tracopower are known names. Not cheap. There are others.

Whatever you choose. Read the data. There are often recommendations for external filtering, or usage restrictions.

[cuivenion]

Cheers David.