Hi all.
I use SMPS inverters (with smps chargers built in) in my work.. I setup UPS systems in homes, offices...etc. I'm using inverters like those in this website: http://www.dc-ac.com/app_com.htm

In my country (Lebanon), electricity goes off for long periods and many times in a day.. say there is 12/24 electricity service (4 hours ON, 4 hours OFF... and so on). A problem with these inverters is that their charger's max. current is only 10A which is not enough for the battery to charge in short periods (4 hours or so).

I'm thinking to use a battery charger in parallel but I'm not sure if this will be ok for the inverter or not.

Any ideas?
Thanks in advance,
Hazim

 

note that the battery charger that I may use in parallel is not smps type.

Regards,
Hazim

 

A gasoline generator would be more practical.

 

A gasoline generator would be more practical.

Right. They are used, there is also "complications"? with huge fuel oil generators, but fuel is expensive here.. UPSs are also used alot and they are a good solution for many cases... Conventional UPS systems (not smps type/those with bulky transformer) are made here with poor electronic design. they have many disadvantages and problems in contradiction with the ones I'm working with except the charging problem.

Regards,
Hazim

 

Hazim,

It is very likely that the battery within the UPS is completely floating, and may be at AC mains potential either when charging, or discharging (via inverter). This could be a very dangerous situation if you then connect external equipment to the battery (for additional charging).

You also need to appreciate that the battery charging rate may be already at the maximum level specified by the battery manufacturer - especially given that you say the charger draws 10A from the AC mains. Charging batteries at a higher rate than specified will quickly damage the battery, especially in your situation where the battery is starting its recharge from a very 'flat' condition.

Maybe you should use the UPS only to support loads for shorter times, such that the battery does not deeply discharge, and hence is more likely to fully recharge when the mains comes back on next.

Ciao, Tim

 

It is very likely that the battery within the UPS is completely floating, and may be at AC mains potential either when charging, or discharging (via inverter). This could be a very dangerous situation if you then connect external equipment to the battery (for additional charging).

I don't think this is happening in my case. The battery terminals doesn't make electrical shock for me if I touched while charging (or discharging). I have to be sure if the battery is in floating state and if there's a way to increase the charging current (using another charger + something else..diodes?)

You also need to appreciate that the battery charging rate may be already at the maximum level specified by the battery manufacturer - especially given that you say the charger draws 10A from the AC mains. Charging batteries at a higher rate than specified will quickly damage the battery, especially in your situation where the battery is starting its recharge from a very 'flat' condition.

I've considered these things... 10A (max.) charging is low for 200Ah battery and even for 150A..

Maybe you should use the UPS only to support loads for shorter times, such that the battery does not deeply discharge, and hence is more likely to fully recharge when the mains comes back on next.

Yes. But when electricity service becomes worse, different solutions arises... Actually, UPS systems are widely used here in Lebanon, as I said in the first post, the UPSs used here are poorly build and they have no protections...

If these inverters I'm using have a 20A charging current then they'll be much much better than those ones used here.. and even cheaper.

Regards,
Hazim

 

Hazim,

You shouldn't be playing with DC voltages if your concept of safety is to touch a battery terminal and see if you get a shock.

The 10A you refer to is the AC mains current draw, not the battery charging current level.

Have you checked what level of mains AC current can be provided, as 10A is a typical maximum level.

 

Hazim,

You shouldn't be playing with DC voltages if your concept of safety is to touch a battery terminal and see if you get a shock.

The 10A you refer to is the AC mains current draw, not the battery charging current level.

Have you checked what level of mains AC current can be provided, as 10A is a typical maximum level.

I couldn't find where Hazim said that 10A was mains current.

 

Thanks Ron. Apologies Hazim - I misinterpreted your initial comment.

Hazim, can you identify the number of batteries/cells in the UPS. 200Ah is a largish battery, even at 2V cell level.

The issue with a floating battery is that when the inverter is operating, the battery may (depending on configuration) be at large transient voltages with respect to earth - and unless the charger is double pole isolated from the battery during that time, then the charger secondary must be suitably designed, including all safety aspects of the charger.

 

Hazim.. what U are attempting is not safe and if u try it, I am sure u will be arrested for blasting ur own house .

If u like to charge a 200AH battery than U have to design an inverter with a rapid charger tht can quick charge a 200AH battery with in Hrs. This my friend is not trivial. U have to be very good at building stuff if u need to to do this.

I suggest that U buy a pure sine wave inverter. This will help a lot

 

Sorry I don't have enough time now for replying on all of you, and for clarifying more.. but R!f@@ it seems you understood the problem which I'm facing, but I need an easy and not much expensive solution for the problem not a radical (and expensive) solution...
But as long as you mentioned, will a pure sine wave inverter charge the battery at higher currents?

 

...it seems you understood the problem which I'm facing, but I need an easy and not much expensive solution for the problem not a radical (and expensive) solution...

If there was such a simple and cheap solution, you wouldn't be here asking for it now.

The answer is simple, but the ears do not like it.

There is no SIMPLE and CHEAP solution to this. There are solutions, but they are either expensive, or they are very complicated. Most times they are some of both.

Do you want to keep chasing a dream, or do you want to start working (and saving$) towards making a solution?

For dependable battery inverter service you need reliable input. Solar is intermittant. Wind is intermittent. Mechanical needs maintenance times.

A real solution won't be easy, but will be more reliable and equipment friendly(possibly) than the current power situation is.

 

You are making it more complicated... Solar and wind energy...
"Cheap" in this case doesn't mean 5 or 10$. A 20A battery Charger can be purchased for something about 40$ or so, this is ok if I used an inverter without internal charger, and using this 20A charger. This maybe a good solution. Also, is it impossible to find an smps inverter like with higher charging current in the market?
I wrote this thread for suggesting and discussing a good and practical solution for the exact problem I presented and not the general problem or for other comments...

Regards,
Hazim

 

The internal resistance and actual output voltage of your chargers will differ slightly.

If you simply connect two or more in parallel, one will 'hog' the action. This usually leads to early failure of each charger in domino fashion.

The remedy is to swamp the internal resistances with a series resistor (power type) in series with each charger.

The internal resistance will be about 0.01 - 0.05 ohms. So a 0.1 - 0.2 ohms resistor in series with each charger should do it. I will leave you to work out the required power ratings for the sharing resistors.

 

I bought this Parallax 40 Amp charger to replace the one in my motorhome that died. It costs about $155 plus shipping. Parallax makes a large variety of chargers with various amperage ratings. I'm sure there are other brands available at competitive prices.

 

I'm starting to think about using to separate parts, inverter and charger. An inverter without charger (inside it) will be for sure cheaper.. and using for example a 20A charger will be a good idea if the overall price doesn't exceed the price of the inverter (with charger in it) very much.
studiot, considering I'm using 0.1 ohm resistor, for the inverters charger the max. current is 10A, then 10^2*0.1=10W, a 20W resistor should be used. For a 20A charger, the power rating of the resistor should be 20^2*0.2=80W, doubling will become 160W!!!
This isn't a good idea I see

Regards,
Hazim

 

studiot, considering I'm using 0.1 ohm resistor, for the inverters charger the max. current is 10A, then 10^2*0.1=10W, a 20W resistor should be used. For a 20A charger, the power rating of the resistor should be 20^2*0.2=80W, doubling will become 160W!!!
This isn't a good idea I see

Not quite.

The higher the current capacity of an individual charger, the lower its internal resistance is likely to be.

So you do not want 0.2 ohms for a 20 amp one, if you require 0.1 for a 10 amp. the reverse is more likely to be true.
You should also remember that 10 amps times 0.1 ohms means that you will loose 1 volt in the resistor, so you will have to increase the charge voltage to compensate.
Remember also that you require 1 resistor per charger.

 

Using a Rapid charger and a separate Inverter is more efficient, easy and reliable.

U can connect or configure them in a way that the charger is disconnected during inverter operation and bypass the inverter, and with it reconnects the charger to charge the battery with in the time u need.

This way u can buy or make a charger specific to ur battery capacity.
Hell!! this is a better way since the charger and inverter are separate, you a less likely to loose when one goes bad. Either way only the charger or inverter dies, shud anything happen

I believe u have an inverter. So, why not concentrate on the charger to Ur battery bank. Use a uC or PIC to switch them automatically. This is a way too easy approach

 

Using a Rapid charger and a separate Inverter is more efficient, easy and reliable.

U can connect or configure them in a way that the charger is disconnected during inverter operation and bypass the inverter, and with it reconnects the charger to charge the battery with in the time u need.

This way u can buy or make a charger specific to ur battery capacity.
Hell!! this is a better way since the charger and inverter are separate, you a less likely to loose when one goes bad. Either way only the charger or inverter dies, shud anything happen

I believe u have an inverter. So, why not concentrate on the charger to Ur battery bank. Use a uC or PIC to switch them automatically. This is a way too easy approach

This is exactly what I'm thinking about. I'm thinking to do that when using 150Ah or 200Ah battery especially. Now I'm buying the inverters with internal charger, but I'll look for Inverters without charger and use a separate rapid charger with contactors if needed. When the electricity goes off, the charger stops charging (its power is off) and the inverter starts... this is not a problem, and I see no need for a PIC or uC.

I still have a small question about "rapid" charger. These battery chargers that are made simply from a transformer and a rectifier (two diodes) and a board for automatic charging, are they considered "rapid" chargers? I thing yes since they starts at high current and ends at low current before charging stops. Also there are smps type chargers available with charging current of 20A and maybe more in an electronics store I buy from, these usually have constant charging current right? then they are not "rapid" chargers? Also, which type are (in general) more expensive?

Regards,
Hazim

 

Please, any information about smps battery chargers? are 20A smps chargers starts at high (20A?) current and lowers with time till battery if fully charged? or it charges with constant current? Also what about their prices compared to ordinary chargers? (those with bulky transformer and rectifier and controlling board)

Regards,
Hazim