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One for the car/bike battery charging pros..

-alan-

-alan-

pfm Member
I have one of the budget micro-processor controlled/multi-programme chargers which can - in theory at least - vary the charging current to suit the current state of the battery.

I've been using it in 'trickle-charge' or what I think is supposed to pass for one on a small 12V bike battery using the motorbike/smaller capacity battery setting. It seems to bring the battery up to charge fairly well, and then hold it at 14.1V indefinitely.

- Problem is that if you leave it at that setting for a longer period of time, within a couple of weeks the electrolyte levels drop to the point where the plates are uncovered, which can't be good. Kind of defeats the object of a fit-and-forget tricks-charger if its boiling off the electrolyte methinks ?

D'ye think I have a faulty charger - or battery - or is the drop in electrolyte level just part and parcel of charging an old-style lead-acid battery ?
 
14.1V is a bit heavy and will evaporate electrolyte. I have done this with a dumb charger, I just set it on a timer for an hour a day. No boiling off, no dead battery. If it needs it, it gets it. If not then it has an hour a day.
 
Perhaps bring battery up to full charge then store. After next use, repeat?

Rookie here so could be wrong but has worked well with my seasonal boat batteries.
 
That's what we do with our "banger bike".
It's a very old semi-auto scooter with a sidecar, we use it for transporting awkward stuff or even doing the local DIY shop.
Aquired it as a total rotting heap with a good engine and have slowly made it whole again.

We use it only a couple of times a month at best so the battery needs a little help.
 
It isn't evaporating or "boiling off".
By applying a reverse voltage above the normal output voltage, you are operating an electrolysis cell - recharging is just a different electrolysis reaction. The water is being broken down into hydrogen and oxygen. A simple fact of science.

The surprise is that at 2V over, the loss of electrolyte isn't faster.

A lead-acid battery in good condition, kept dry, will hold charge for literally years - been there, done that. The battery must be dry and clean externally and ideally stood on something like a plastic mat or fertiliser bag. If there is a conductive path between the poles, even of (very) high resistance, the battery will discharge over extended storage.
 
Get one designed for a bike. I have an Optimate, designed to be left on over the winter and also charged up a large dead battery on my Jag
 
I think the issue may with the charger in this case. AFAIU the charger should top out at 13.8V.

I would look for one designed for small batteries, or with a motorcycle battery setting. I had a couple bought from Lidl which had a motorcycle setting. I used them on my two ride-on mowers mainly over the winter. Batteries only needed a check yearly, which fine considering the poor regulation of the charging systems on mowers.
 
cctaylor said:
AFAIU the charger should top out at 13.8V.
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Irrelevant except that it would electrolyse the water 1.8/2 times as fast.

If the voltage applied is over the 12V output of the battery, it will electrolyse the water, simple fact of science.
 
ks.234 said:
Get one designed for a bike. I have an Optimate, designed to be left on over the winter and also charged up a large dead battery on my Jag
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you're right about the loss of water being electrolysis. Less so the overvoltage, a fully charged LA battery is 13.6V, this being 6 x 2.3, and a LA cell being 2.3V.

I've never kept a battery charged for more than a couple of months, however claen and dry it is. However a monthly charge works fine. The problem is that you forget, and if you leave them discharged for a month or more they are useless. That's why battery shops have elaborate charging records.
 
I had a nearly new spare sat in my garage for something like 2 years and it was absolutely fine when swapped onto my car. Started the car with ease, but I did not check the voltage. I was put wise to the clean etc., especially standing on an insulator (not damp concrete or the like), by my local motor spares shop owner - the battery will be fine for years he assured me.

6 x 2.3? 13.6?

In practise, a commercial LA battery that has seen some use puts out a little over 12V - 12.2, 12.3 - that sort of range.
 
A proper intelligent charger should take the battery to 13.8V and then drop to very low current in the mA range. At these currents the electrolyte should not evaporate.

A lead acid battery which is not regularly charged to full capacity is at risk of sulfation which can permanently reduce the capacity and life span of the battery.
 
cctaylor said:
A proper intelligent charger should take the battery to 13.8V and then drop to very low current in the mA range. At these currents the electrolyte should not evaporate.
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Incorrect. If you apply a reverse voltage above that of the battery, it will electrolyse the electrolyte. Inescapable simple scientific fact.
I also checked @steve57 number for cell voltage as I had a recollection from Physics lessons back in the 1970's that it was 2.1V, which indeed it is, not 2.3V. Which is why a LA battery seldom puts out more than about 12.3 or so volts - a number repeated on battery manufacturers' websites.

cctaylor said:
A lead acid battery which is not regularly charged to full capacity is at risk of sulfation which can permanently reduce the capacity and life span of the battery.
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True, but only so when a battery isn't charged fully. If stored correctly, fully charged, a battery will not suffer this to any significant degree.
 
Thanks Vinny / thanks also to all for your comments and input.

I have a fancier Optimate unit as well. With two bikes and a car in semi-storage, I'd hoped rotating the chargers between the machines would be an easy way of keeping all the batteries in reasonable fettle and always ready for use without having to go to the bother of connecting/disconnecting them

That particular charger has a motorbike setting - which is the one that holds it at 14.1V. Not sure what current it puts through it at that point. The Optimate unit doesn't have a bike/small battery setting per-se, but claims to be smart enough to handle whatever you throw at it. Probably worth checking what it does current wise once it thinks the battery is at full charge.

A question - will simply putting a multi-meter in the charging circuit in series give me a reasonably accurate picture of what's happening with the current, or will the fact of having inserted another electrical device with its own resistance/impedance w.h.y. possibly confuse the charger logic and end up with a different steady-state current flow ?
 
-alan- said:
Thanks Vinny / thanks also to all for your comments and input.

I have a fancier Optimate unit as well. With two bikes and a car in semi-storage, I'd hoped rotating the chargers between the machines would be an easy way of keeping all the batteries in reasonable fettle and always ready for use without having to go to the bother of connecting/disconnecting them

That particular charger has a motorbike setting - which is the one that holds it at 14.1V. Not sure what current it puts through it at that point. The Optimate unit doesn't have a bike/small battery setting per-se, but claims to be smart enough to handle whatever you throw at it. Probably worth checking what it does current wise once it thinks the battery is at full charge.

A question - will simply putting a multi-meter in the charging circuit in series give me a reasonably accurate picture of what's happening with the current, or will the fact of having inserted another electrical device with its own resistance/impedance w.h.y. possibly confuse the charger logic and end up with a different steady-state current flow ?
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An ammeter has an extremely low resistance so will be effectively invisible in the circuit. This includes a multimeter set to amps. Feel free to connect it, the battery charger clips probably have a resistance bigger than that of a 10A multimeter.

Make sure your meter will safely measure a couple of amps. If you connect a meter set to mA and put 1A or more through it, it might just never read again. Modern digital meters are pretty tough, but the old style AVO meters were often killed thus way.
 
An essential 'gotcha' to add to @stevec67 's note - ensure you reset the leads on your DVM back to read volts (usu the +ve goes into a different socket from reading current - before you then use it to check battery voltage and experience the fun of applying a near-dead-short across a car battery, and instant smoke from the test leads (BT.DT...)

(Incidentally the current shunt internal to such meters will survive just fine - its usu. a thick steel wire about 30mm long, good for >20a - its; everythign else that will give up around it.You may be saved by a 10 or 20A internal fuse in the meter...)


Re: SLA voltages - here is some data from a reputable source/ what I've always used - applies to modern 'valve-regulated' types of vehicel starting/lighting/charging batterys

SLA terminal voltage capacity.PNG


AFAIU You need to get above 2.16v/cell to initiate any charge - regardless of the state of charge of the battery (ie even when 'flat'); 'float' charge on any SLA is 13.6-13.8Vdc, at which point the current will drop to mA to maintain charge and life (the standard setting in things like fire alarm panels for a guaranteed min 5yr battery life) - and at this , on modern valve-regulated cells, you will not loose electrolyte. Up to 14.4v for maximum charge rate; you'll find cars when running will default to about 14.1v (SLA) up to 14.3-14.4v (for absorbed-mat 'gel-cell' SLAs) - but only when above idle, for emissions /efficiency reasons; Even my old bmw deliberately drops to 'float' voltage under 1000rpm; neat when you think about it - just enough,not wasting power driving lots of charge into a battery, but guarantees the alternator still picks up any incidental loads since its output terminal voltage is always above the battery on its own. Cars in the last 5yrs or so often have alternators running a pwm outputs for even greater efficiency gains/ease of management; one reason some cars require you to let the car know ('code') what kind of battery a new replacement is.
 
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That relationship ties in reasonably well with a table of resting voltage vs. charge capacity I pulled off t'net some years ago:

Battery Voltage - State of Charge:

12.66v . . . . . . . . . . 100%
12.45v . . . . . . . . . . 75%
12.24v . . . . . . . . . . 50%
12.06v . . . . . . . . . . 25%
11.89v . . . . . . . . . . 0%
 
Alan - I like the figures you suggest; in all such things there's a bit of temperature dependency - which is why I think your numbers are realistic for the UK when my graph was a US source : the difference between (UK) 18-20degC vs 25degC (US) ambient temp expectation, and a rise of 2mV/degC or so explains the difference. Looks reasonable - my alternator's output is a up to 100mV higher on warm days than cool ones (as it will be for any intelligent charger)
 
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