Charging 4x 12v 7.2amp in serious 48v

[bernardgreen]

I have 4x 12v 7.2 batteries

sensible maximum current is going to be about 5 amps but any decent charger is going to self-limit.

5 Amp charging into a 7.2 Amp Hour battery would be OK only if the battery was capable of being fast charged, if they are not designed for fast charging then 5 amps will damage them.
As mentioned before.... check the battery's data sheet about charging rates

 

[Jackrae]

5A for a 7.2Ah lead acid battery is a bit on the high side (assuming your "7.2amp" actually means 7.2Ah)
Lead acid charge rates should normally be limited to C/7 so in your case 1A would be kinder on the batteries.

 

[ericmark]

One normally charges at 1/10 of amp hour rating so 0.72A or 720mA. With valve regulated lead acid be it gel or absorbed glass mat the charge voltage is very important normally float charged at 52.8 volt for a 48 volt battery. There are special chargers stage and pulse designed to charge quicker. The pulse measures voltage between pulses and is used where the battery is used while being charged and the stage charger measures the current and as it drops it also drops the voltage. With the stage charger it needs matching to battery amp hour rate. So for example one designed for a 60 to 110 amp hour battery would drop to final stage when current dropped to 5A.

To recharge in 12 hours it would need to be a stage or pulse charger. The valve in a valve regulated 60 AH battery is very different to the little rubber cap used in a 7.2 AH version. So although charging a 3A or 56.8 volt until charge rate drops to 1A then dropping to 52.8 volt may seem in line with the larger batteries in the main the small VRLA batteries are only float charged at 52.8 volt for 48 volt battery and it would take maybe 2 or 3 days to fully charge. In the main they would be used for things like UPS and these will only kick in with a power cut. So although one could build a 48 volt stage charger it is not really going to work with such small batteries.

The pulse charger is quite complex and not really a DIY project. So in real terms you have to charge as 12 volt and use as 48 volt. As to if in parallel or as individual batteries that's really down to charger used. The C-Tek charger comes in many outputs, but they are expensive, Lidi and Aldi both from time to time do cheap versions. So at 0.8A you would likely need one per battery and at 10A one for all batteries in parallel.

If it were me I would start with one small one all batteries in parallel and if not big enough then two and charge in pairs the Lidi one when on offer is around £14 and 3.8A max so if me I would get the C-Tek MSX 3.8 (Around £80) to start with which means nearly same output as Lidi when they next come on offer. The cheaper 3.3A is for 14 ~ 75 AH where the MSX is 1.2 ~ 80 AH that is the difference between stage and pulse charging. It all depends on how long between being used you have to recharge and how discharged the batteries become. From memory the Lidi charger started at 4.2 Ah so in theroy you could use 4 Lidi chargers one on each battery for a quick re-charge. But £14 to £80 is a huge step is it worth waiting for next Lidi special offer?

With open vented lead acid if charged in parallel you would need an equalizing charge every so often, it is common to use 160 Ah batteries in parallel as over that size too heavy to lift. But with the small VRLA battery life is only around 2 years. One has to accept they need changing every two years, larger VRLA as used in mobility scooters can easy have a 10 year life. But not the small ones sorry.

 

[bernardgreen]

Charging batteries in parallel is not reccomended. In fact connecting batteries in parallel is never a good idea. If one cell in a battery fails then the good battery will discharge through the defective battery. Depending on the type of failure the current from good battery to bad battery may be high enough to damage them and / or create a hazard of boiling battery , fumes and maybe fire.

it is acceptable to use four 12 volt chargers to charge a 48 string of cells provided that the chargers ouputs are totally isolated from each other.



Care needs to be taken that the batteries are evenly charged and that all chargers are on at the same time. Rotating the batteries ( or chargers ) monthly will reduce the effects of differences in charging rates between chargers.

 

[JohnW2]

Charging batteries in parallel is not reccomended. In fact connecting batteries in parallel is never a good idea. If one cell in a battery fails then the good battery will discharge through the defective battery.

That, in itself, can obviously be prevented by the use of diodes - by that doesn't alter the fact that the appropriate charging voltage (or the charging current if the voltage is fixed) will differ between 'good' and 'bad' batteries.

Kind Regards, John

 

[SimonH2]

I'm wondering what amps I should be putting into them I don't want to destroy them ?

Assuming these are "dry" cells, aka AGM ...
Get the manufacturers data sheets and they'll have charging data for different operating regimes. For your usage, it'll be classed as cyclic - and you'll need a multi-stage (aka intelligent) charger.

The normal charge cycle is a current limited (aka constant current) bulk charge regime until the battery voltage reaches a threshold point. The charger then switches to constant voltage while the charge current reduces. When the charge current drops to a specified threshold, the charger switches to a lower constant voltage (float mode).
Matching the current and voltage points to the battery is critical to long battery life - overcharging results in excess gassing, this causes loss of water from the electrolyte, and in an AGM battery there is very very little water to lose. The various values should also be temperature compensated.

A quick search for 'multi stage agm charger "48V"' or 'intelligent agm charger "48V"' throws up a number of results - they do exist.
Now, for a DIY project, bear in mind that quite a few computer UPSs are 48V ...

 

[JohnW2]

The normal charge cycle is a current limited (aka constant current) bulk charge regime until the battery voltage reaches a threshold point. The charger then switches to constant voltage while the charge current reduces. When the charge current drops to a specified threshold, the charger switches to a lower constant voltage (float mode). Matching the current and voltage points to the battery is critical to long battery life - overcharging results in excess gassing, this causes loss of water from the electrolyte ...

Indeed - but, as has been pointed out and discussed, the more cells one is charging in series, the greater the chance that one or more will be 'duff' (or, at least, less than fully healthy). If that happens, then a 'voltage point' which would be appropriate for, say, 24 x healthy 2V cells in series might well be result in 'higher than appropriate' voltages across most of the cells (hence the potential problems to which you refer).

Kind Regards, John

 

[SimonH2]

Yes, and I've witnessed this first hand more than once. It is "interesting" when this happens with largish parallel strings - especially when the system is nominally 120V and the system doesn't have any detection for faulty strings.
Our current UPS at work uses multiple sets of 7Ah batteries, but it does have sensing on each string (individual current and temperature). If one goes bad, it'll load the batteries to drop the voltage and stop the excess current. The old UPS we had didn't have this - it would just keep floating the batteries and when we had a bad cell, well the smell from cooking lead-acid batteries is "quite distinctive"

 

[ericmark]

You make a good point one bad cell be it series or parallel can result in over charging other cells. With larger batteries chargers tend to have a time limit as well as current monitor, but with smaller batteries this tends to be missing.

The question as to how long the batteries how to recharge has not been answered. http://sterling-power.com is mainly working with far larger batteries to the 7Ah talked about here. But the charts they show comparing pulse charging with stage charging still holds good.

What we have to remember a 7 Ah battery will cost something like £15 to spend £60 on a charger to prolong it's life is therefore going a bit silly. So 4 x £15 still only £60. To build a charger giving a float charge voltage of 53 volt with a maximum current of 1A and have two sets of batteries is likely more cost effective than using either pulse or stage chargers.

If one swaps battery pack rather than recharge the one battery pack then likely you have 24 hours rather than 12 hours to recharge.

The idea of warnings should a cell become short circuit will save destroying 4 batteries when one goes faulty. But question is for a £15 battery is it worth making a monitoring system? I would think a faulty cell would likely be apparent on the discharge cycle and in real terms monitoring for a faulty cell on charge cycle is not required.

An electric bike using lead acid is a cheap option. To worry about failed cells with a cheap system is maybe going OTT. If using NiMh batteries then one is looking at some serious vehicle, but lead acid is more just A to B.