my son has a 125cc atv. i would like to up size the battery to add a few lights and add to the reliability of it starting in cold weather.

i plow a residential area and my son does shoveling. i figured if i can get it running better he can ride that around the area instead of walking. he could also keep some supplies on it also.

i picked up a set of 4 led amber strobes im going to put on it to increase his visibility

 

Upsizing the battery isn't going to change the output capacity of your stator and voltage regulator to keep the bigger battery charged. Ultimately the battery needs to be kept charged overall, and that has to be done by the battery charging system - which has a maximum output current. If you add more electrical loads to your quad, and those loads, plus the existing loads, exceed your battery charging system capacity then you will be discharging your battery while your riding no matter what size battery you're putting in. A bigger battery will go dead slower, but it is still going dead while your riding.

An exception to this is if you have heavy short term electrical loads, following by light (normal) loads, such that the "average" load current is below your battery charging system capacity so that the battery averaged over time keeps charged. A winch is a good example of this. It is used rarely, but when it is used the current draw is enormous (like 80 amps). In this case a bigger battery will allow the winch to be used a little longer before you have to stop and let the battery get charged back up. But a bigger battery, after running it down, will also take a lot longer to charge back up. Energy out must be replaced by the equivalent energy in.

Another exception is if you "plan" on exceeding the charging system capacity, and accept that you're draining the battery while you're riding around. Then it is a case of calculating how big a battery you need to safely make it through the day (with a safety margin), and then come up with an external charging system to plug in at night to top the battery back off.

Cold cranking amps capacity is larger on a bigger battery. Do you have slow cranking when really cold (but not at warmer temps)? And does that slowness go away when you jump the quad to your car or truck? If jumping to your car fixes the slow cranking, then the next obvious question is whether you need a bigger battery, or you current battery is just bad and needs to be replaced.

LED lights are tremendously efficient compared to incandescant lights (measured in required current for the same output lumens). So any additional loads would have the least impact if they were LED. But LED lighting requires that your lighting is DC powered (some lighting systems are AC powered). Do you know which you have?

 

Well since i am hoping to add a 2000lb winch to my 150 i am using an old Land Rover battery that i managed to fit under the seat. But as Lynn has said the stator wont output enough to charge a larger battery so i keep my batter on charge after every ride. Although this may be bad for the battery it doesnt really effect me since its just a junk battery anyway.

 

If it's starting hard, often times it needs a valve adjustment.