Double battery chargers





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  1. #1
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    Default Double battery chargers

    Good morning,
    Question: will connecting two similar battery chargers to a battery double the rate of charge? Will it be safe and can it damage either or both of the chargers or the battery or both?
    Will connecting a battery charger permanently to my second battery which is charged by the car's alternator (obviously not at the same time) while travelling, cause damage to the charger due to the alternator's charging the battery?

  2. #2
    4ePikanini Guest

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    No

    Under no circumstances should you connect two charging systems (similar or not) to one battery without the necessary systems in place.

    Read and study to understand
    http://www.hilux4x4.co.za/dual_battery/index.php

  3. #3
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    Quote Originally Posted by 4ePikanini View Post
    No

    Under no circumstances should you connect two charging systems (similar or not) to one battery without the necessary systems in place.

    Read and study to understand
    http://www.hilux4x4.co.za/dual_battery/index.php
    .
    .
    Are you sure !

    There is no problem connecting one mains charger to your battery.
    and then to speed up the charging, without going into the 30= amp region, it is safe to connect a second mains battery charger.

    Just ensure the 2 positives from the chargers are connected to the positive of the battery, and obviously, the 2 negatives from the battery charger to the negative post.

    All you are doing is raising the charge voltage at the battery terminals, and this in turn raises the amps pumped into the battery, for want of a better word.

    Just switch off the chargers before you disconnect from the battery, as you really dont' want to cause a spark, and ignite the hydrogen gas.

    'vette

  4. #4
    4ePikanini Guest

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    It can be done but as stated you need to do the necessary research and get the appropriate equipment.







    As taken from http://www.mpoweruk.com/chargers.htm



    Charging Times

    During fast charging it is possible to pump electrical energy into the battery faster than the chemical process can react to it, with damaging results.
    The chemical action can not take place instantaneously and there will be a reaction gradient in the bulk of the electrolyte between the electrodes with the electrolyte nearest to the electrodes being converted or "charged" before the electrolyte further away. This is particularly noticeable in high capacity cells which contain a large volume of electrolyte.
    There are in fact at least three key processes involved in the cell chemical conversions.

    • One is the "charge transfer", which is the actual chemical reaction taking place at the interface of the electrode with the electrolyte and this proceeds relatively quickly.
    • The second is the "mass transport" or "diffusion" process in which the materials transformed in the charge transfer process are moved on from the electrode surface, making way for further materials to reach the electrode to take part in the transformation process. This is a relatively slow process which continues until all the materials have been transformed.
    • The charging process may also be subject to other significant effects whose reaction time should also be taken into account such as the "intercalation process" by which Lithium cells are charged in which Lithium ions are inserted into the crystal lattice of the host electrode.

    All of these processes are also temperature dependent.

    In addition there may be other parasitic or side effects such as passivation of the electrodes, crystal formation and gas build up, which all affect charging times and efficiencies, but these may be relatively minor or infrequent, or may occur only during conditions of abuse. They are therefore not considered here.

    The battery charging process thus has at least three characteristic time constants associated with achieving complete conversion of the active chemicals which depend on both the chemicals employed and on the cell construction. The time constant associated with the charge transfer could be one minute or less, whereas the mass transport time constant can be as high as several hours or more in a large high capacity cell. This is one of the the reasons why cells can deliver or accept very high pulse currents, but much lower continuous currents.(Another major factor is the heat dissipation involved). These phenomena are non linear and apply to the discharging process as well as to charging. There is thus a limit to the charge acceptance rate of the cell. Continuing to pump energy into the cell faster than the chemicals can react to the charge can cause local overcharge conditions including polarisation, overheating as well as unwanted chemical reactions, near to the electrodes thus damaging the cell. Fast charging forces up the rate of chemical reaction in the cell (as does fast discharging) and it may be necessary to allow "rest periods" during the charging process for the chemical actions to propagate throughout the bulk of the chemical mass in the cell and to stabilise at progressive levels of charge.

    A memorable though not quite equivalent phenomenon is the pouring of beer into a glass. Pouring very quickly results in a lot of froth and a small amount of beer at the bottom of the glass. Pouring slowly down the side of the glass or alternatively letting the beer settle till the froth disperses and then topping up allows the glass to be filled completely.

    Hysteresis
    The time constants and the phenomena mentioned above thus give rise to hysteresis in the battery. During charging the chemical reaction lags behind the application of the charging voltage and similarly, when a load is applied to the battery to discharge it, there is a delay before the full current can be delivered through the load. As with magnetic hysteresis, energy is lost during the charge discharge cycle due to the chemical hysteresis effect.

    Fast charging also causes increased Joule heating of the cell because of the higher currents involved and the higher temperature in turn causes an increase in the rate of the chemical conversion processes.

    The section on Discharge Rates shows how the effective cell capacity is affected by the discharge rates.
    The section on Cell Construction describes how the cell designs can be optimised for fast charging.

  5. #5
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    You guys always spoil a good debate with the facts..............
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  6. #6
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    Bravo HenriS!
    4ePikanini, that is more than several mouthsful! I read the thread you referred to and I must say that I have safety systems in place. My question was really asked because I have two small chargers(identical, so called "smart" chargers) rated at 3.5amps. I thought that connecting them to my aux. battery at the same time may then work far better than one at a time.
    Also, will leaving it connected, i.e. when the 220v power supply to the chargers (or only one charger for that matter)is switched off cause problems, either with the charger or the battery or both? What implications are there? At the moment I have a charger installed on the back of my bakkie which I use to maintain the battery when camping with 220v available. I disconnect the charger from the battery when the charger is not in use, but will keeping it connected to the battery when the 220v power to the charger is switched off, have an effect? Will the charge from the alternator damage the charger if it is not disconnected? Worse still, can it damage the alternator?

  7. #7
    4ePikanini Guest

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    Quote Originally Posted by dudley View Post
    Bravo HenriS!
    4ePikanini, that is more than several mouthsful! I read the thread you referred to and I must say that I have safety systems in place. My question was really asked because I have two small chargers(identical, so called "smart" chargers) rated at 3.5amps. I thought that connecting them to my aux. battery at the same time may then work far better than one at a time.
    I am no expert and sometimes these things are greek to me as well.
    If the chargers are really smart they will probably 'see' each other and ultimately only deliver the 3.5 amp charge until the battery is charged.

    If they do not and deliver the double charge on a battery that cannot take it you might get damage the battery or worst case scenario an explosion.

    If you want to 'quick' charge the battery then rather get a charger and battery combination that is designed to 'quick-charge'

    Quote Originally Posted by dudley View Post
    Also, will leaving it connected, i.e. when the 220v power supply to the chargers (or only one charger for that matter)is switched off cause problems, either with the charger or the battery or both? What implications are there? At the moment I have a charger installed on the back of my bakkie which I use to maintain the battery when camping with 220v available. I disconnect the charger from the battery when the charger is not in use, but will keeping it connected to the battery when the 220v power to the charger is switched off, have an effect? Will the charge from the alternator damage the charger if it is not disconnected? Worse still, can it damage the alternator?
    With the charger connected to battery while in use it probably ends up being the same as the alternator charging the battery while in use so I don't see a problem there (provided it is an intelligent charger and doesn't overcharge)

    If you disconnect the 220v but leave the charger connected to battery (ie. charger is not charging) and then start the car, I'm sure there will be no problem.

    The way I see it with it connected and car off the battery is sending 12v back to the device and when the car is running it will send 13v-14v back to the charger which the charger should be able to cope with, but don't take my word for it.

    If the 220v is connected to the charger and the car is running that ends up being the same as 2 devices charging. Depending on the 'intelligence' of the charger and alternator, they might 'see' each other again and adapt relative to each other in order for the battery to get the correct voltage.

    Whatever course you take be sure about it and test it good as you don't want a failed/exploding battery.

  8. #8
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    Default

    A "Smart" battery charger does not use a standard transformer. Most of the time they use what is known as a switch mode power supply(High Frequency eg. Computer psu, Most consumer electronics). the basic idea behind the "Smart"charger is that it will monitor the voltage and depending on that it will either charge "fast" or trickle charge. With two charger on same bat. one of two things can happen.
    1. Two will Charge double the standard rate(depending on monitor circuit).
    2. The two chargers will start to oscilate(the one will charge at the standard rate the other will trickle because it is "seeing" the other battery charger and not the battery in other words they will see-saw on and off all the time.)

    If you use two normal transformer chargers they will work together if they have the same output voltage and amparage rateing.

    As a general rule don't connect two Chargers/Power supplies together.

    That said my PC uses two 1000W psu's

    Edit: if you leave it connected it sould not cause a problem. most have a reverse polarity protection diode on the output

    You can try connecting the two on the same battery. Just use a fuse on each charger rated at the chargers max current. If one fuse blow they will not work together.
    Last edited by Ebot; 2009/06/15 at 03:18 PM.

  9. #9
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    Many years ago, when PC PSU's were 150w, I had a server 450w PSU die on me. It was a UNISYS machine. I was told that they could only import a PSU and that would take 3weeks minimum. So I took 2 x 250w PC PSU's and coupled them in parallel. That box lived another 3 yrs until the server was scrapped.
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  10. #10
    4ePikanini Guest

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    Quote Originally Posted by Ebot View Post

    That said my PC uses two 1000W psu's
    Hopefully not connected to the same battery

    OT : check out a google server using 12v battery as UPS HERE

  11. #11
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    Connecting two chargers to a battery is really nothing new. Just observe some simple rules of thumb.

    In general charging current should not exceed 10% of battery capacity.

    When charging keep an eye on temperature so it does not exceed 40 degC(Ideally keep temp as near to 25degC as possible)

    Match the charger output voltages.

    I regularly connected 6 "intelligent" 50A switchmode power supplies to battery banks for Telkom, some places took as much as 12. It is all about the application
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  12. #12
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    Quote Originally Posted by Henris View Post
    You guys always spoil a good debate with the facts..............
    .
    .
    That's why I try to keep it simple.

    We can all copy and paste pages, but to the 'lay man' does he understand it.

    No.

    'vette

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    Quote Originally Posted by uk_vette View Post
    .
    .
    That's why I try to keep it simple.

    We can all copy and paste pages, but to the 'lay man' does he understand it.

    No.

    'vette
    In lay man's terms listen to the youngster he has done his homework and knows what he is talking about. In case of a small error it is you not a big corporation that is going to pay for it
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  14. #14
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    Thanks guys. I will use the fuse method to test the setup. The battery is 75A/h deep cycle so the two chargers together should be OK. If the fuses go bang, I shall let you all know! I thought of also putting a simple in-line switch between the chargers and the battery to make sure that nothing befuddles something else.

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