lead acid battery charge-discharge

Following on from the thread about second use for old electric car batteries: surely as end of life is reached for any cell the self discharge increases and thus a higher proportion of the charge gets expressed as heat?

I need to provide hot washing water in one of our vans, about 10 litres is enough and these basin units are readily available for £250.

I can see a split charge system will only charge the second battery when the main vehicle battery exceeds 13.5V, and even a facility I'm told that will only charge it on overrun. This will get the water hot on the run to work.

During the day how does one prevent the auxiliary battery discharging below 50% DOD, is it by voltage sensing or do VA have to be integrated?

AJH

Reply to
news
Loading thread data ...

On Sunday, April 13, 2014 9:24:29 AM UTC+1, snipped-for-privacy@sylva.icuklive.co.uk wrot e:

if thats whats causing its end of life.

If you mean electric power, you'd get far more heat for far less by harvest ing from the exhaust, or even the engine coolant.

NT

Reply to
meow2222

Really? Why? Surely it'll charge whenever the engine is running?

Tim

Reply to
Tim+

It depends on the split charge relay in use. Some just connect the batteries in parallel with the engine running, some sense the charge on the starter battery, and connect intelligently.

As for how do you stop the leisure battery discharging too far, it's done by sensing the battery voltage. As all lead acid batteries have the same (Roughly) discharge curve against the percentage of available charge used, even ones which are approaching their end of life, there's no need to calculate amp hours used and put in.

These problems were all solved long ago by the boaters and motorhome owners.

One slightly more expensive way to get quicker hot water is to use a well insulated calorifier heated by the engine, which has a high power alternator fitted to run an small immersion heater as well. If you have room, that is.

Reply to
John Williamson

The amount of power a lead acid battery stores is insignificant compared with its weight For any heating load,you wouldn't want to be carting a big enough one around. As someone else says, there is far more energy available from the engine cooling system or exhaust.

Reply to
harryagain

OK what modes of failure are there and how do they manifest themselves?

I'm talking about 150W element heating 5 litres of water through 45C, the unit costs ~£250 and requires 27Ahr from cold which takes about an hour. I don't know how well it is insulated to keep warm during the shift. So I may get more heat from the exhaust or a diesel heater but the capital cost wouldn't warrant it.

As it is a tractor battery (644) will easily manage the demand but the issue is will it get charged enough in the average commute (about 40 minutes). The split charge facility protects the vehicle battery as it only closes above 13.5V but I need something to inhibit the heater if the auxiliary battery falls below 50% DoD or about 12.2V.

AJH

Reply to
news

No it won't.

Nowadays 'split charge relays' are usually 'voltage sensing relays'. In other words they are controlled by the voltage available at the vehicle battery rather than by a connection to the 'sense' wire on the alternator. This is much more simple and easy than the old method. (Bear with me by the way). When the alternator is running and working properly and the main battery is reasonably charged the voltage rises and the relay senses this and connects the auxiliary battery. It is important that the relay is connected to the vehicle charging circuit at a point where there is negligible resistance between it and the vehicle battery, otherwise it might sense a high voltage and switch when the vehicle battery is not charging properly. To put it another way, part of the installation should be a check of the main battery terminals, ground connection, and wiring.

Some voltage sensing relays also monitor the voltage of the aux battery before connection, in case it is dangerously low. If it is the relay won't connect it to the vehicle battery and alternator. This is a good precaution. If there's a fault on the aux circuit you don't want the alternator output shorting out just as you leave harbour/ hit the M1. This might be a problem for your application. Certainly if the heater was connected and drawing 27A there would be a risk that the relay wouldn't switch.

I doubt if you'd be able to apply a cut-off at 12.2V because if you were drawing 27A the battery terminal voltage would very quickly be below that (although it would recover quite a lot once the load was removed). If your need to protect the aux battery from total discharge (and you do if using a voltage sensing relay because most of these won't connect the aux battery if it is totally flat) I suggest a cut off of about 11.1V for a high load like 27A. You might be able to get a conventional voltage sensing relay that will adjust down to that value and use it in series with the heater supply. Look at the specs. These are the ones I use:

formatting link

Alternatively use a purpose made item:

formatting link
£108 though...

Bill

Reply to
Bill Wright

rote:

lead deposits shorting plates ionic contamination causing self discharge broken lead plate sulphation electrolyte too low electrolyte too weak electrolyte too strong electrolyte dried out terminal corrosion plate buckled & shorting likely others I'm not thinking of just now

esting from the exhaust, or even the engine coolant.

Why would one spend 250 on a diesel heater when heat can be harvested from the exhaust with metal & rubber pipe?

opamp or comparator, relay. If you can do electronics.

NT

Reply to
meow2222

Thanks for your concise and complete post Bill, this device still needs a continuous duty solenoid but looks a good way to go even if the vehicle may need a bigger alternator.

AJH

Reply to
news

Because running the engine creates noise and pollution which many people find annoying. Because it's cheaper to run a diesel powered heater than the engine used for moving the vehicle. Because the diesel powered heater starts heating the water immediately without first having to heat many kilogrammes of metal. Because with today's efficient diesels, they take ages to start getting warm enough to make a difference to the water temperature in a calorifier.

However, it's the electric water heater that's costing £250, a diesel heater costs over £500 fitted. Installing a calorifier run off either exhaust heat or engine cooling water costs even more.

Reply to
John Williamson

Hell, my diesel *car* has a diesel powered heater.

Reply to
Huge

And I'll bet it needs it for the first ten miles or so.

Reply to
John Williamson

In message , snipped-for-privacy@sylva.icuklive.co.uk writes

Voltage sensitive relay as used on caravans these days?

Reply to
bert

In fact the vans already have eberspacher hot air units but as you say the capital cot of changing to ones that heat water is prohibitive.

AJH

Reply to
news

rom the exhaust with metal & rubber pipe?

I wont quibble with the noise. Its the coolant circuit that takes time to h eat up though. Most heat is lost via exhaust, not coolant.

How would it cost over 500 to bend some pipe and tack it on to the manifold ?

NT

Reply to
meow2222

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.