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Thread: How to extend UPS backup time

  1. #1
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    Default How to extend UPS backup time

    Lets start with the Objective


    1.To increase the backup time of your UPS
    2.Maintain all the batteries in good shape. (Ensure similar discharge and recharge)
    3.Make sure all the batteries (internal & external) are properly charged.


    This mod can be applied to any UPS system regardless of different battery configuration.


    I will briefly cover the whole concept and will respond to queries and concerns as and when required.


    The UPS internal battery is connected to a AC powered timer relay.
    The AC powered timer relay also connects to the external battery bank and an external battery charger.
    When the relay is powered, it will connect the battery to the external battery charger.
    But when power goes out the relay looses power and connects the external battery bank to the internal UPS battery in parallel.

    Why you need this mod:

    The typical UPS battery mod uses internal UPS charging circuit to charge the external battery. In majority of the case, the internal UPS charger cannot the charge the external batt bank to full extent when the Amp hour capacity is too big.

    This also affects the internal battery as well since this battery also can not receive a full charge. Eventually you will get poor runtime and premature failure of both internal and external batteries.


    A small calculation should help you to understand the phenomena.


    In typical case, UPS can not provide more than 1 A to charge the batteries. 1A is good for up to 9A Battery.


    So charge time is roughly 12.6 hours for fully discharged 9A battery at 1A charge. (Assuming 40% efficiency loss)

    Now lets say if you want to add additional 75amp Battery bank which then bumps up the total capacity to 84A then charge time is 117.6 hours.

    Required components:

    1. At least 50A VDC contact rating timer relay with AC coil.
    or a simple double pole single throw switch available at local hardware store but you have to press the switch manually whenever u need to connect the external bank to the UPS.

    2. 2 Gauge power wire.
    3. Solder Gun+solder (optional)
    4. An external 3 stage smart charger or Constant voltage power supply that can put out 13.6 V( for 12V UPS system) or 27.2 V (24 UPS system)with at least 10A

    All of the above can be purchased from patuatuli or stadium in a single day.

    Challenges:
    I'm currently running the above setup with low contact rated relay and looking for a higher rated relay. So, anyone works with relays or contactors or know places that sells high powered relays or contactors pls let me know.

    DISCLAIMER: I won't be liable for any damages resulting from the use, misuse, or abuse of the information contained herein, nor for collateral damages resulting from the use, misuse, or abuse thereof. This includes but not limited to equipment failures, lost data, fires, or injuries. You use this modification at your own risk.

    You will be exposed to very high current setup with 24VDC(or more if you have a setup with higher DC bus voltage) which is still safe since 24VDC won't be able to break human body's internal resistance. This is the reason why we don't feel electrocuted when we touch battery terminals.
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    Last edited by minitt; November 15th, 2014 at 23:32.

  2. #2
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    ok, I just want to know about the approx. cost to get this? Can u gimme an idea?
    Why so Serious!

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    The major cost is the battery and the charger. Each 75A battery costs 6500 taka by hamko. 24v Ips machine can serve as a charger. Around 5k. Remember bigger battery need bigger charger.Or u can seperately buy a charger from patuatuli moon complex. Each power relay 120A 30VDC contact rating cost 1400 (i have one extra let me know if u want that) 2gauge wire depends on ur need. One Ac timer relay 250 taka nobabpur.

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    made couple of changes to the original design but the overall principal is still the same. A 4 pole double throw toggle (on/off) switch is added which is used in parallel to the main relay to strengthen connection hence reducing the voltage drop.

    if anyone has basic understanding of how electricity works and willing to give it a try do let me know. I'll try my best to guide you or u can visit my place if you want to see it in person.
    Last edited by minitt; December 28th, 2014 at 23:44.

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    Maan what's up with u and ups!

    - - - Updated - - -

    Btw did u just paralled two different rated batteries :o :o whaat!

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    Quote Originally Posted by redwan hasan View Post
    Maan what's up with u and ups!

    - - - Updated - - -

    Btw did u just paralled two different rated batteries :o :o whaat!
    The internal batteries are charged with built-in charger of the UPS and external batteries get their charge externally. Both battery set (internal 9A and external 75A) do not talk to each other during charging. ( physically isolated by the Power relay)

    During battery mode ( internal & external batt are in parallel), the whole load should be proportionally supported by the both sets.

    In this case, I have total 84Ah Capacity

    Lets assume that my load is 200 watt then 10.7% of the load( 21.4 watt) should be supported by the internal battery and the remaining 89.28% or 178 watt should be drawn from the external bank.

    I've went through several attempts to observe the internal battery discharge status during power cut. The ups was running for more than 1+ hour and when the power came back the internal battery still shows majority of it's charge because it was the external bank that took the hit.
    Last edited by minitt; December 30th, 2014 at 02:24.

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    Quote Originally Posted by minitt View Post
    The internal batteries are charged with built-in charger of the UPS and external batteries get their charge externally. Both battery set (internal 9A and external 75A) do not talk to each other during charging. ( physically isolated by the Power relay)

    During battery mode ( internal & external batt are in parallel), the whole load should be proportionally supported by the both sets.

    In this case, I have total 84Ah Capacity

    Lets assume that my load is 200 watt then 10.7% of the load( 21.4 watt) should be supported by the internal battery and the remaining 89.28% or 178 watt should be drawn from the external bank.

    I've went through several attempts to observe the internal battery discharge status during power cut. The ups was running for more than 1+ hour and when the power came back the internal battery still shows majority of it's charge because it was the external bank that took the hit.
    if i am not wrong, in this case you can use an ups whose internal battery gives little to no backup at all, and you can use 2 to more number of external batteries connected in parallel way to each other in place of single 75A if you want to.

  8. #8
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    Quote Originally Posted by minitt View Post
    The internal batteries are charged with built-in charger of the UPS and external batteries get their charge externally. Both battery set (internal 9A and external 75A) do not talk to each other during charging. ( physically isolated by the Power relay)

    During battery mode ( internal & external batt are in parallel), the whole load should be proportionally supported by the both sets.

    In this case, I have total 84Ah Capacity

    Lets assume that my load is 200 watt then 10.7% of the load( 21.4 watt) should be supported by the internal battery and the remaining 89.28% or 178 watt should be drawn from the external bank.

    I've went through several attempts to observe the internal battery discharge status during power cut. The ups was running for more than 1+ hour and when the power came back the internal battery still shows majority of it's charge because it was the external bank that took the hit.
    In depth question why did that happen? Probably the external one had a slightly higher Voltage rating.

    Btw i dont think its a good practice to use different batteries in parallel as they might have different internal resistance and discharge curve.

    U can use multiple relay with voltage trip points that will allow the ups to switch between different batteries.

    Im actually working on one with a micro controller unit, hope i will be post everything in my blog, pretty simple, u can make one for urself too.

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    Quote Originally Posted by redwan hasan View Post
    In depth question why did that happen? Probably the external one had a slightly higher Voltage rating.

    Btw i dont think its a good practice to use different batteries in parallel as they might have different internal resistance and discharge curve.

    U can use multiple relay with voltage trip points that will allow the ups to switch between different batteries.

    Im actually working on one with a micro controller unit, hope i will be post everything in my blog, pretty simple, u can make one for urself too.
    i would love to see it once u r done. Seems interesting. But i do agree that both set will have diff resistance and discharge curve. But what could be the worst impact due to these diff? Pls be as much descriptive as possible.

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    Quote Originally Posted by minitt View Post
    i would love to see it once u r done. Seems interesting. But i do agree that both set will have diff resistance and discharge curve. But what could be the worst impact due to these diff? Pls be as much descriptive as possible.
    Well two banks have different voltages even very slight difference, there will be circulating current, meaning the current will circulate within the banks and it might be dangerous if the high current bank has higher voltage. It will try to charge a 9Ahr battery at 75A, might cause permanent damage to the battery.

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    When i unplug the main cable to put the ups on battery mode the external bank initial voltage drops to 24.8. So how can a 25v batt damage/overcharge the internal batteries which also initially hovers around 24.6v and both banks are putting out electricity in same direction and busy supporting the load.

    As a matter of fact many online ups vendor provides external battery bank with ext charger and in that setup both set of batt(int n ext) discharge together with no ill effect.

    but i do understand that it could be a possible phenomena. Do u have any academic reference that i can look into?

    if ur concerns were valid both of my internal batteries would've been fried by now. What do u think?

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    Quote Originally Posted by minitt View Post
    if ur concerns were valid both of my internal batteries would've been fried by now. What do u think?
    I don't think they will be fried. But all of the batteries would have their life drastically reduced, because they are basically going through multiple micro cycles for every charge cycle.

    - - - Updated - - -

    Quote Originally Posted by redwan hasan View Post
    Well two banks have different voltages even very slight difference, there will be circulating current, meaning the current will circulate within the banks and it might be dangerous if the high current bank has higher voltage. It will try to charge a 9Ahr battery at 75A, might cause permanent damage to the battery.
    I don't think slightly different voltage will give up 75A, you have to factor the internal resistance of both batteries. If the higher bank doesn'tcross the float charge voltage the current should not be higher than 0.1-0.2C of the smaller banks.

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    But how come ups vendors like eaton apc prolink provide similar setup with ext batt n charger where both banks contribute in parallel to hold the given load while not suffering the phenomena we r talkig about.

    - - - Updated - - -

    Quote Originally Posted by aaz_swapnil View Post
    I don't think they will be fried. But all of the batteries would have their life drastically reduced, because they are basically going through multiple micro cycles for every charge cycle.

    - - - Updated - - -



    I don't think slightly different voltage will give up 75A, you have to factor the internal resistance of both batteries. If the higher bank doesn'tcross the float charge voltage the current should not be higher than 0.1-0.2C of the smaller banks.
    your 2nd point makes sense.

    I'm still a bit skeptical to yours and @redwan hasan point (current circulation).

    Is there any test that I can try to be certain?

    Try this link which says its ok to have similar setup like the one i have
    Last edited by minitt; December 30th, 2014 at 20:25.

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    It's not prohibited to connect batteries in parallel. It's just not recommended. If the batteries have different internal resistance, under load the cell voltage drop across the batteries will be different, thus causing a current flow within the batteries which is not part of the full circuit current flow. That wastes a bit of the capacity. And also when the batteries are partially discharged, the larger batteries have higher charge and voltage available, causing more current flow among them even with charger and load turned off, consuming part of the charge cycle of both batteries.

    Connecting batteries in parallel is common in RC communities with Li-Po batteries. But they do this while considering the above matters. The trade offs there are justified by the need of more current (more than 80 amps in 16v).

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    Quote Originally Posted by aaz_swapnil View Post
    It's not prohibited to connect batteries in parallel. It's just not recommended. If the batteries have different internal resistance,( ext big bank with low resistance, int small bank high resistance) under load the cell voltage drop across the batteries will be different( agreed), thus causing a current flow within the batteries which is not part of the full circuit current flow. probably yes but I'm still on the look out for a good credible source that confirms this electricity circulation. The PDF that I have provided doesn't mention this effect and rather recommends it under certain parameters.

    That wastes a bit of the capacity. ( yes only if the above really occurs) And also when the batteries are partially discharged, the larger batteries have higher charge and voltage available, causing more current flow among them( Higher voltage battery will always contribute more than the internal smaller bank during load) even with charger and load turned off (If u noticed my design the ext batt and int bank are only connected when the UPS runs on battery. Both the batt bank charges separately and while charging there is no physical contact , consuming part of the charge cycle of both batteries. (not possible in my design since both banks take their charge separately and there is no physical connection between them.)

    Connecting batteries in parallel is common in RC communities with Li-Po batteries. But they do this while considering the above matters. The trade offs there are justified by the need of more current (more than 80 amps in 16v).
    have a look at the PDF link that I have provided. Let me know what you think.
    Last edited by minitt; December 30th, 2014 at 21:19.

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    I think this PDF assumes ideal conditions and availibility of professional monitoring.

    Also,

    Click image for larger version. 

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    and

    Click image for larger version. 

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ID:	35362

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    Quote Originally Posted by aaz_swapnil View Post
    I think this PDF assumes ideal conditions and availibility of professional monitoring.

    Also,
    and

    Good find on the 1st line that says The ratio of 1:5 is optimal for practical standpoint. In my case its actually 1:8 i think. Correct me if I'm wrong. (9A int and 75A ext)

    So, if I want to ensure both bank have same voltage readings while they r on load, I have to put some sort resistor in series that brings down the voltage of external battery to the the level that is equal to the voltage of internal bank?


    For 2nd snap, Doesn't seems that much relevant in this case. This is for swapping the the bad battery while not turning off the load.

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    Quote Originally Posted by minitt View Post
    Good find on the 1st line that says The ratio of 1:5 is optimal for practical standpoint. In my case its actually 1:8 i think. Correct me if I'm wrong. (9A int and 75A ext)

    So, if I want to ensure both bank have same voltage readings while they r on load, I have to put some sort resistor in series that brings down the voltage of external battery to the the level that is equal to the voltage of internal bank?


    For 2nd snap, Doesn't seems that much relevant in this case. This is for swapping the the bad battery while not turning off the load.
    Series resistors will be constant values. The load and voltage drop will change often. And also the internal resistance will change over time.

    BTW, I'm just being bit OCD-ish. I don't think you'll get much problem with your setup though.

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    Quote Originally Posted by aaz_swapnil View Post
    Series resistors will be constant values. The load and voltage drop will change often. And also the internal resistance will change over time.

    BTW, I'm just being bit OCD-ish. I don't think you'll get much problem with your setup though.
    but still if there is any way to fine tune it, I'm willing to go for it. @redwan hasan was talking about switching back and forth between batt banks based on voltage readings (hope i got the concept right) but that will induce lag( in ms) and may/will eliminate the 0 sec transfer advantage of having a online UPS.

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    1. Well that won't be 75A but should be high enough to damage the smaller banks capacity.
    2. Sadly i can only provide the same info for transformers right now which are very much different from batteries. But the circulating current is a real thing, higher voltage bank will both charge the batteries and supply load current.
    3. Yes you can parallel batteries but everywhere u will see that only similar type having same capacity is paralleled. Open up a laptop battery u will see 3/4/6/8 18650 cells are there. Its a good practice to go with this.
    4. Ups that supports external batteries, well im pretty sure they just simply don't parallel with the internal and external one.
    5. Relays are pretty fast like 20us or even faster, so u don't have to worry about it.

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