Dual battery problem- advice please

[Smacka14]

One of my 2 batteries has died. They are linked together, one a 75 amp hr, the other a 90 amp hr. If I replace the 75ah, (the one that has died) should I replace it with another 90 ah? or is it ok to mix it with a battery of a different amp hour? Thanks

[lethal098]

not going to matter.

[FNQCairns]

Go with another 90, although your old 90 might be a match for a new 75 by now but still I always choose to use like and like but yeah it doesn't matter you are only making one big battery, how big is up to you.

cheers fnq

[TOPAZ]

It is necessary for you to tell us how the batteries are "linked" - simply parallelling two batteries, even of the same size, will result in most of the charge from your motor going to the better battery, (due to differences in internal resistance), which is not want you want. You must have them connected via a 4-position switch (OFF-BAT1-BAT2-BOTH) to manage which battery is being used and which battery is being charged. You can upgrade to an automatic system which will charge one battery until 'full', then switch the charging current to the second battery.
Two batteries connected in parallel, will discharge by themselves without any external current draw, as one battery will always have a higher internal resistance than the other (albeit small) The flatter battery has the higher resistance.


Richard

[Smacka14]

Hi Topaz, I do have a 4 position switch. So do you still recomend two batteries of the same ampage? Thanks

[TOPAZ]

Hi Smacka,

It should make no difference if the two batteries are not of the same capacity, provided you are aware that one has more cranking power than the other, and take that into account when deciding which battery to use for starting, and which for accessories, or however you manage them.
The only problem which arises with these 4-way battery switches, is if you expect to be able to charge BOTH batteries from the motor at the same time with the switch in the BOTH position. One battery will ALWAYS take most of the charge, due to differences in internal resistance, and unfortunately, this will always be the battery that needs charging the LEAST!

Richard

[MarkDiver]

You could opt to fit a Voltage Sensitive Relay (VSR) at around $125 which will automatically sense and charge the main battery first until full and then switch over and charge the other until full and then remain charging both. Easy installation.

[the gecko]

Ive got the VS relay switch too, they work great. When the main battery is full, it then charges the second battery while under way.

Andrew

[Spaniard_King]

You could opt to fit a Voltage Sensitive Relay (VSR) at around $125 which will automatically sense and charge the main battery first until full and then switch over and charge the other until full and then remain charging both. Easy installation.

Sorry to put a dampner on the VSR's but they have an issue:-[ If the main battery has a fault and it never can acheive full charge then the VSR will never charge the 2nd battery... Speaking from experince.. not shooting from the hip::)

[cormorant]

Some larger motors have dual battery charging capability built in or as an accessory.

Not a VSR fan especially the cheap ones. What marine ones are now avaliable as I have seen corrosion on the ones designed for 4WD? Have seen some VSR set of the voltage alarms on dash guages on some newer motors as well.

Batteries I like to match style, size and brand as most alterrnators are dumb.

[Fed]

It is necessary for you to tell us how the batteries are "linked" - simply parallelling two batteries, even of the same size, will result in most of the charge from your motor going to the better battery, (due to differences in internal resistance), which is not want you want. You must have them connected via a 4-position switch (OFF-BAT1-BAT2-BOTH) to manage which battery is being used and which battery is being charged. You can upgrade to an automatic system which will charge one battery until 'full', then switch the charging current to the second battery.
Two batteries connected in parallel, will discharge by themselves without any external current draw, as one battery will always have a higher internal resistance than the other (albeit small) The flatter battery has the higher resistance.


Richard

All the batteries I've had have the reverse characteristics to yours Topaz.
IE: Flat batteries have a lower internal resistance than charged batteries.;)