DIY cranking or booster battery pack

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white exec
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Re: DIY cranking or booster battery pack

Post by white exec »

Doubt that the running current would exceed that figure, although there might be an instantaneous starting current a tad higher, which could be ignored. Possible that an analogue meter wouldn't even register the initial surge, except by way of a substantial needle flick!
Make sure you get a d.c. clamp-on ammeter, and not one of the usual electricians' a.c. (inductive) ones.

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Re: DIY cranking or booster battery pack

Post by MikeT »

Initially, I only had the SLA cca spec as a maximum current draw to aim for but I've since learnt I'm comparing apples to oranges in that the SLA suffers voltage drop so the amps will increase in proportion, whereas Life cells discharge while holding their voltage stable.

Yes, any ammeter I use needs to capture data or I might miss it!

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Re: DIY cranking or booster battery pack

Post by MikeT »

My LifePO4 cells safely arrived today from Hong Kong after a few false starts. I'm now waiting on the jump leads and various adapters/connectors to make up the booster pack (though they may take a little while longer as they're on the slow-boat). Meanwhile I'll start testing these cells for capacity and cell balance before they get put into service, just in case I need to raise a claim with the vendor.

Not having much luck finding a reasonably priced DC ammeter.
In doing an ebay search I found this https://www.ebay.co.uk/itm/DC-0-90V-0-1 ... 2223687278
Although it doesn't offer the versatility of a clamp-on ammeter, it ranges up to 1000A (with a suitable shunt) and (with a suitable relay) does appear to offer typical BMS functions needed for a permanently fitted alternative power source. :!:

And for a spot of relative light relief - this nutter makes me laugh and can be informative too.


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Re: DIY cranking or booster battery pack

Post by white exec »

I'm a bit concerned about the idea of a 1000A-capable shunt, and especially at that all-in price.
I have experience of several of these Chinese meter modules (eg those by DEOK and DROK, extensively on Amazon) and they are very good.

A 1000A shunt is actually just an incredibly low and high-wattage resistance, across which the digital meter reads the voltage drop (and hence the current). As such, connections to the shunt - difficult enough anyway - are crucial to the measurement accuracy, and a few milli-ohms either way of those connections would affect the reading.

So I remain sceptical about it. In any case, the shunt/device would need calibrating at heavy-current levels, and how would you do this? But it is cheap, so maybe a bit of fun.

I did find this:
Sperry DSA1020TRMS, d.c. clamp meter:
http://www.sperryinstruments.com/en/sea ... rd=DSA1020
Won't be cheap. Not sure about vendors.

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Re: DIY cranking or booster battery pack

Post by MikeT »

The shunt (and cut-off relays) are not included in the price, Chris. But that's really for part 2 of this project, assuming part 1 is successful. :D
Yes, I saw ElecticBoom using a 600A DC clamp on (HH2002) True RMS but the pricetag is too high for a single function requirement and he says they're not so accurate either.

Otherwise I'm going to have to assume that unless the cranking battery has depleted drastically, the Life cells won't be stressed as a booster. If in doubt, I'll "slow-charge" the car with my LiPo's first. I'll do anything to avoid lugging that SLA indoors again! [-X

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Re: DIY cranking or booster battery pack

Post by white exec »

Just thinking that if the main cranking battery is in decent shape, the booster cells might not bring much to the party - or are you hoping to assess how much each contributes? Or is the aim to be able to start the car with a more-or-less flat normal battery?

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Re: DIY cranking or booster battery pack

Post by MikeT »

I just want a battery that will start the car when I want it started :lol:

This is all experimentation but it at least gives me trickle/maintenance charge as a minimum with the possibility of boost cranking an otherwise non-starter.

If it effectively can replace and outlive the SLA, then all the better.

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Re: DIY cranking or booster battery pack

Post by MikeT »

I mentioned earlier about comparing apples to oranges due to the difference in discharge curves and it's still not clarifying in my head.

For instance, I know my battery reads 9-10V during cranking and starts quickly. Let's say I measured 600A at the time.
What should I expect from a battery that maintains 12V during cranking?

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Re: DIY cranking or booster battery pack

Post by white exec »

Crudely theoretically, 20% more current taken (another 2V, or 20% of the 10), but motors are non-linear (non-ohmic) things...*
You could always jump another good standard car battery across the first (as a test) and see whether the 9-10V improves.

* The faster they spin, the more back emf (reverse voltage) they generate, iirc. #-o

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Re: DIY cranking or booster battery pack

Post by MikeT »

(Theoretically speaking) More current? As in it will still measure 600A but the motor will spin faster and presumably start even faster?

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Re: DIY cranking or booster battery pack

Post by white exec »

If it spins faster, it will be taking more current. It's current that generates the strong magnetic flux that powers the motor under load, and gives it torque. More torque will be needed to crank the engine faster, so that means more current.

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Re: DIY cranking or booster battery pack

Post by MikeT »

Put another way, if I can easily start my car with 600A @10V, should I expect the same from just 500A @12V?

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Re: DIY cranking or booster battery pack

Post by white exec »

If you apply 12V rather than 10V, the current through the motor will be higher, not lower.

Although a d.c. brush motor doesn't behave resistively/linearly like a fixed resistance, if you pretend it does for a moment, then it's a bit like applying voltage to a light bulb: the higher the voltage, the brighter the lamp, and the greater the current that flows through it.

Forget voltage. It's just a way of creating current through something: 'pressure' if you like.
In a motor (or electromagnet), current is responsible for creating a magnetic field. The stronger the field, the more power (torque) the motor develops.

If your engine turns over easily with 10V, it will turn over quicker with 12V, because the current through the motor is greater.

You have a choice: power it with 10V (and get current X), or with 12V (and get current Y).
Y will always be greater than X.
You can't straightforwardly push one up and the other down!

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Re: DIY cranking or booster battery pack

Post by EDC5 »

Exactly, the motor is a fixed resistance load (basically) rather than a fixed power load.

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Re: DIY cranking or booster battery pack

Post by MikeT »

white exec wrote:
28 Oct 2018, 17:16
If you apply 12V rather than 10V, the current through the motor will be higher, not lower.


Just as well I didn't buy an ammeter :lol: