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 »

Mike, I may be being thick (happens!) but I still remain puzzled about this project.

I can understand that the normal car battery could be letting you down - self-discharge, poor starting current, not holding charge, etc etc, all the usual elderly battery things.
I can understand your not wanting to lug it indoors to continually recharge it, or run charger cables out to the car.

SFAI understand it, your aim is to provide the original big battery with a source of on-car back-up charging (to bring/keep its voltage up), and also to have on-tap an additional source of battery capacity to aid starting. OK, understand that.

Decent starting could be effected by
- replacing the old car battery
- adding a second car battery in parallel (à la Range Rover and emergency/military vehicles)
The second would be bulky and difficult, appreciate that.

Then there is the question of recharging.
Both your big battery and the small LiFePO4 cells are recharged from the alternator.

Are you saying in your write-up that the set-up works ok because the small cells are
1: accepting alternator charge quickly - more quickly than the big battery?
2: transferring charge to the big battery when the engine is switched off?

Tell me where I've misunderstood.
Also a question: what is the Ah capacity of each of those two cells?

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Re: DIY cranking or booster battery pack

Post by MikeT »

I don't make much sense at the best of times and I'm really suffering from my efforts so far, especially getting that bloody tracker out of the C3, that was the final straw that laid me up until now, even so I'm still sore as hell and not concentrating too well so please don't think I'm being flippant in my response.

All I did was test a portable charger and power source Chris, using much of what I've had lying around for years - namely multicopter LiPo packs (they're 5000mAh each @14.8V nominal) and accessories. That's what you see in the photos Ellis asked for. It doesn't recharge itself off the alternator or anything else at this point and I have no intention to test that function either as it's LiPo not LifePO4. All in all, it may not seem much of an achievement to anyone else but personally speaking it's already solved several problems in one and I'm very grateful for that solution at least.

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Re: DIY cranking or booster battery pack

Post by white exec »

Thanks, Mike. That joins the dots much better, now understanding that the additional kit is intended as a 'portable charger'.

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Re: DIY cranking or booster battery pack

Post by MikeT »

Yep, when/if I have to buy a replacement cranking battery I don't want it prematurely ageing due to infrequent car use so I consider needing a portable charger as a bare minimum.
When all the other parts arrive the new LifePO4 cells will take over as it's power source as they're more powerful and better suited to being recharged off the alternator plus I'm hoping they can double-up as a cranking supplement.

One thing that occured to me during the charge testing was the 5A limit of the charger. The alternator isn't restricted to 5amps so what upper limit is safe?

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Re: DIY cranking or booster battery pack

Post by white exec »

If you look at articles on the web (wikipedia a good place to start) for the type of battery in hand, the recommended charging rates (float, normal, maximum...) are usually stated. The figure appears something like "C/10", which means battery capacity (in Ah, or mAh) divided by 10 . . . so a 5Ah battery's normal charge rate (to avoid shortening battery life) would then be 0.5A (500mA).

Different types of battery have very different recommended charge rates, so worth checking.

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Re: DIY cranking or booster battery pack

Post by MikeT »

Sorry, I meant what's a safe rate for charging the car's lead acid battery - mine's 75Ah? I have a 6A charger as well but if it'll take more, I'll get a higher rate charger.
I think I remember reading gel batteries should be charged at no more than C/2.
Lithium's should be charged at 1C for optimum life though can take 2C for a fast charge without risk.

I was reading Battery University and it claims engine alternators never achieve a full charge. Hmmm.

Following the top-up charge on the C5, over 24 hours later the resting voltage is now showing 12.4V, a 3 point increase so it's done the battery some good and does suggest BU is correct!

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Re: DIY cranking or booster battery pack

Post by EDC5 »

Well, a lead acid is usually subjected to a no-mercy ~14.3V from the moment the engine stops cranking. I imagine the initial charging current could be many tens of amps.

I know that sealed lead acids are more sensitive to charging current as they can’t gas-off like flooded cells can.

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Re: DIY cranking or booster battery pack

Post by MikeT »

From Wiki "As long as the charging voltage stays below the gassing voltage (about 14.4 volts in a normal lead–acid battery), battery damage is unlikely, and in time the battery should return to a nominally charged state."
But we only give it as much time as the engine is running and depending on the DOD, will cause damage because...

"Starting batteries kept on a continuous float charge will suffer corrosion of the electrodes which will also result in premature failure. Starting batteries should therefore be kept open circuit but charged regularly (at least once every two weeks) to prevent sulfation."

Batteries don't die they're murdered.
Yet we think we're buying "maintenace-free" batteries? :lol:

I saw How's It Made showing Exide in the US recycling the lead to make new batteries from. Apparently, the bulldozers buckets get erroded by the acid and need replacing every six months at huge cost. Perhaps it's more accurate to say we "rent" batteries.

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Re: DIY cranking or booster battery pack

Post by white exec »

The CTEK intelligent battery chargers allow lead-acid batteries to be left on charge continuously, without damaging them. This seems to be achieved by constant monitoring of the battery's state, and automatic adjustment through several charging sequences. Lots of interesting detail on the CTEK site. I use their MXS3.8 here on the XM.

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Re: DIY cranking or booster battery pack

Post by MikeT »

I think Marc might be interested in one of those - it's boasts you "can fit and forget" :D

Getting back to that DROK 1000A power meter that requires a shunt, I found a variation that comes with a hall sensor for current. Aside from a hall sensor not requiring direct contact, which is more reliable/desirable?

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Re: DIY cranking or booster battery pack

Post by white exec »

Most 'non-contact' d.c. ammeters use the steady magnetic field radiated from the conducting cable to produce a reading in the clip-on (or clamp) meter.

Hall-effect is something else: it is the movement of a piece of ferrous (magnetisable) metal, passing across/near a sensing coil (or coil wrapped round a magnet), and producing an induced electrical pulse as it passes. ABS sensors are one example, as are speed/distance sensors on a gearbox, or TDC sensors on a flywheel. Tachometers are usually triggered by such rapid pulses; as the frequency of pulses increases, so does the reading on the instrument.
Hall-effect sensors cannot respond to a constant d.c. current - they need pulses or change (or a.c.) to operate.

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Re: DIY cranking or booster battery pack

Post by GiveMeABreak »

I’ve got an AGM battery fitted to mine, so even had to buy a charger that was AGM compatible. I suppose I would have to check up on something like that too to see if it’s compatible. =P~

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Re: DIY cranking or booster battery pack

Post by MikeT »

From wiki, Chris
A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications. In a Hall effect sensor, a thin strip of metal has a current applied along it.
Marc, I know the CTEK I was looking was AGM compatible, can't say if that's across the whole range though.

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Re: DIY cranking or booster battery pack

Post by GiveMeABreak »

At £306 for a CTEK smart pass battery charger, I'll think I'll pass on that one :rofl2: I could get 2 new decent batteries for the price of that. :-D It's not urgent anyway as I can charge mine up on the vehicle (recommended way) so it's good for a winter top up charge.

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Re: DIY cranking or booster battery pack

Post by MikeT »

I'm sure they're worth every penny Marc. The one I looked at boasted a 2 year warranty.
Then I saw an article from some 10 years ago when they had 4 year warranty. :lol: